Time to First Byte: What It Is and Why It Matters

Written by Harry Roberts on CSS Wizardry.

I’m working on a client project at the moment and, as they’re an ecommerce site, there are a lot of facets of performance I’m keen to look into for them: load times are a good start, start render is key for customers who want to see information quickly (hint: that’s all of them), and client-specific metrics like how quickly did the key product image load? can all provide valuable insights.

However, one metric I feel that front-end developers overlook all too quickly is Time to First Byte (TTFB). This is understandable—forgivable, almost—when you consider that TTFB begins to move into back-end territory, but if I was to sum up the problem as succinctly as possible, I’d say:

While a good TTFB doesn’t necessarily mean you will have a fast website, a bad TTFB almost certainly guarantees a slow one.

Even though, as a front-end developer, you might not be in the position to make improvements to TTFB yourself, it’s important to know that any problems with a high TTFB will leave you on the back foot, and any efforts you make to optimises images, clear the critical path, and asynchronously load your webfonts will all be made in the spirit of playing catchup. That’s not to say that more front-end oriented optimisations should be forgone, but there is, unfortunately, an air of closing the stable door after the horse has bolted. You really want to squish those TTFB bugs as soon as you can.

What is TTFB?

The TTFB timing entry isn’t particularly insightful. View full size/quality (375KB)

TTFB is a little opaque to say the least. It comprises so many different things that I often think we tend to just gloss over it. A lot of people surmise that TTFB is merely time spent on the server, but that is only a small fraction of the true extent of things.

The first—and often most surprising for people to learn—thing that I want to draw your attention to is that TTFB counts one whole round trip of latency. TTFB isn’t just time spent on the server, it is also the time spent getting from our device to the sever and back again (carrying, that’s right, the first byte of data!).

Armed with this knowledge, we can soon understand why TTFB can often increase so dramatically on mobile. Surely, you’ve wondered before, the server has no idea that I’m on a mobile device—how can it be increasing its TTFB?! The reason is because mobile networks are, as a rule, high latency connections. If your Round Trip Time (RTT) from your phone to a server and back again is, say, 250ms, you’ll immediately see a corresponding increase in TTFB.

If there is one key thing I’m keen for your to take from this post, its is that TTFB is affected by latency.

But what else is TTFB? Strap yourself in; here is a non-exhaustive list presented in no particular order:

Latency: As above, we’re counting a trip out to and a return trip from the server. A trip from a device in London to a server in New York has a theoretical best-case speed of 28ms over fibre, but this makes lots of very optimistic assumptions. Expect closer to 75ms. This is why serving your content from a CDN is so important: even in the internet age, being geographically closer to your customers is advantageous.

As above, we’re counting a trip out to and a return trip from the server. A trip from a device in London to a server in New York has a theoretical best-case speed of 28ms over fibre, but this makes lots of very optimistic assumptions. Expect closer to 75ms. Routing: If you are using a CDN—and you should be!—a customer in Leeds might get routed to the MAN datacentre only to find that the resource they’re requesting isn’t in that PoP’s cache. Accordingly, they’ll get routed all the way back to your origin server to retrieve it from there. If your origin is in, say, Virginia, that’s going to be a large and invisible increase in TTFB.

If you are using a CDN—and you should be!—a customer in Leeds might get routed to the MAN datacentre only to find that the resource they’re requesting isn’t in that PoP’s cache. Accordingly, they’ll get routed all the way back to your origin server to retrieve it from there. If your origin is in, say, Virginia, that’s going to be a large and invisible increase in TTFB. Filesystem reads: The server simply reading static files such as images or stylesheets from the filesystem has a cost. It all gets added to your TTFB.

The server simply reading static files such as images or stylesheets from the filesystem has a cost. It all gets added to your TTFB. Prioritisation: HTTP/2 has a (re)prioritisation mechanism whereby it may choose to stall lower-priority responses on the server while sending higher-priority responses down the wire. H/2 prioritisation issues aside, even when H/2 is running smoothly, these expected delays will contribute to your TTFB.

HTTP/2 has a (re)prioritisation mechanism whereby it may choose to stall lower-priority responses on the server while sending higher-priority responses down the wire. H/2 prioritisation issues aside, even when H/2 is running smoothly, these expected delays will contribute to your TTFB. Application runtime: It’s kind of obvious really, but the time it takes to run your actual application code is going to be a large contributor to your TTFB.

It’s kind of obvious really, but the time it takes to run your actual application code is going to be a large contributor to your TTFB. Database queries: Pages that require data from a database will incur a cost when searching over it. More TTFB.

Pages that require data from a database will incur a cost when searching over it. More TTFB. API calls: If you need to call any APIs (internal or otherwise) in order to populate a page, the overhead will be counted in your TTFB.

If you need to call any APIs (internal or otherwise) in order to populate a page, the overhead will be counted in your TTFB. Server-Side Rendering: The cost of server-rendering a page could be trivial, but it will still contribute to your TTFB.

The cost of server-rendering a page could be trivial, but it will still contribute to your TTFB. Cheap hosting: Hosting that is optimised for cost over performance usually means you’re sharing a server with any number of other websites, so expect degraded server performance which could affect your ability to fulfil requests, or may simply mean underpowered hardware trying to run your application.

Hosting that is optimised for cost over performance usually means you’re sharing a server with any number of other websites, so expect degraded server performance which could affect your ability to fulfil requests, or may simply mean underpowered hardware trying to run your application. DDoS or heavy load: In a similar vein to the previous point, increased load with no way of auto-scaling your application will lead to degraded performance where you begin to probe the limits of your infrastructure.

In a similar vein to the previous point, increased load with no way of auto-scaling your application will lead to degraded performance where you begin to probe the limits of your infrastructure. WAFs and load balancers: Services such as web application firewalls or load balancers that sit in front of your application will also contribute to your TTFB.

Services such as web application firewalls or load balancers that sit in front of your application will also contribute to your TTFB. CDN features: Although a CDN is a huge net win, in certain scenarios, their features could lead to additional TTFB. For example, request collapsing, edge-side includes, etc.).

Although a CDN is a huge net win, in certain scenarios, their features could lead to additional TTFB. For example, request collapsing, edge-side includes, etc.). Last-mile latency: When we think of a computer in London visiting a server in New York, we tend to oversimplify that journey quite drastically, almost imagining that the two were directly connected. The reality is that there’s a much more complex series of intermediaries from our own router to our ISP; from a cell tower to an undersea cable. Last mile latency deals with the disproportionate complexity toward the terminus of a connection.

It’s impossible to have a 0ms TTFB, so it’s important to note that the list above does not represent things that are necessarily bad or slowing your TTFB down. Rather, your TTFB represents any number of the items present above. My aim here is not to point fingers at any particular part of the stack, but instead to help you understand what exactly TTFB can entail. And with so much potentially taking place in our TTFB phase, it’s almost a miracle that websites load at all!

So. Much. Stuff!

Demystifying TTFB

Thankfully, it’s not all so unclear anymore! With a little bit of extra work spent implementing the Server Timing API, we can begin to measure and surface intricate timings to the front-end, allowing web developers to identify and debug potential bottlenecks previously obscured from view.

The Server Timing API allows developers to augment their responses with an additional Server-Timing HTTP header which contains timing information that the application has measured itself.

This is exactly what we did at BBC iPlayer last year:

The newly-available Server-Timing header can be added to any response. View full size/quality (533KB)

N.B. Server Timing doesn’t come for free: you need to actually measure the aspects listed above yourself and then populate your Server-Timing header with the relevant data. All the browser does is display the data in the relevant tooling, making it available on the front-end:

Now we can see, right there in the browser, how long certain aspects of our TTFB took. View full size/quality (419KB)

To help you get started, Christopher Sidebottom wrote up his implementation of the Server Timing API during our time optimising iPlayer.

It’s vital that we understand just what TTFB can cover, and just how critical it can be to overall performance. TTFB has knock-on effects, which can be a good thing or a bad thing depending on whether it’s starting off low or high.

If you’re slow out of the gate, you’ll spend the rest of the race playing catchup.

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