In the world of WordPress there are a lot of different plugins you can install to extend its functionality. One of the most popular plugins is Wordfence, a security plugin developed by the fine folks over at Defiant.

Test the performance implications of your own WordPress plugins with Kernl WordPress Load Testing.

Most WordPress developers understand that the more plugins you add to your site the slower it goes, but exactly how much slower isn’t something that is often measured. More importantly, nobody has bothered to figure out the performance implications of installing many of the most popular plugins available to WordPress users today.

This all changes now with this series of blog posts exploring the performance implications of different WordPress plugins. In this series of posts we’ll test each plugin in isolation and then with caching enabled using Kernl’s WordPress load testing service. First up, is Wordfence.

Machine Setup

The test machine for these tests was a $5 Digital Ocean droplet in their SFO2 (San Francisco) data center. The machine has 1GB of RAM, 1 vCPU, and a 25GB hard disk.

The software installed on the machine is as follows:

Ubuntu 19.04 with all updates installed.

Nginx 1.16.1

PHP-FPM 7.3

MariaDB 10.3

For tests that required caching we used our favorite caching plugin, W3 Total Cache, with memcached as the data store.

The theme that was used was TwentyTwenty with no modifications and the content was an export of this blog.

All traffic was generated out of Digital Ocean’s NYC3 (New York City) data center.

Test Methodology

A series of 4 tests were run to test the performance implications of installing Wordfence. They were:

Each test was with 200 concurrent users for 1 hour.

Maximum Requests per Second

The first metric that we looked at was the maximum requests per second that the site was able to handle under each situation outlined above.

Wordfence – Maximum Requests per Second

As you can see the difference between having Wordfence enabled and having Wordfence disabled is huge. The non-cached site with no plugins enabled handled a maximum of 26 requests per second. With Wordfence enabled it could only handle 12.

More interesting though was that with caching enabled (W3 Total Cache) the site could handle 165 requests per second, but only 67 requests per second with Wordfence enabled.

These results were so surprising that we ran the tests twice. The results were the same (within 1%-2%) each time.

First Error Occurrence

The next metric we looked at was when did the first error occur during our load tests.

Wordfence – First Error req/s

Once again we see that adding Wordfence took a pretty serious toll on our performance. For the baseline test (no plugins, no cache) we see our first error at around 26 requests / second. In the Wordfence test with no cache we saw it at 12 requests / second.

With our caching enabled test, we never saw any errors when Wordfence wasn’t enabled. However with Wordfence enabled we saw our first error at 56 requests / second.

Average Response Time

Now that we’ve looked at server capacity metrics (max requests, first error), let’s take a look at how your end user experience changes with Wordfence enabled.

Wordfence – Avg. Response Time (milliseconds)

These results were fairly interesting and not at all what was expected. The average response time for the baseline test was around 4.6 seconds, while the average response with Wordfence enabled was about 2.5 seconds. So why might that be? Looking through the data it appears that the baseline test had far more successful requests and that with Wordfence enabled the requests seemed to fail faster. In short, baseline test had higher throughput but slower response time. The Wordfence test had lower throughput and faster response time.

Turning our attention to the caching scenarios, we can see that enabling Wordfence is extremely problematic at scale. With caching enabled, our baseline test had an average response time of 146ms. With caching + Wordfence that time ballooned over 10x to 1874ms.

99th Percentile Response Time

Our final metric was looking at the 99th percentile response times for our different scenarios. What does that mean? It answers the question “How long does it take for 99% of requests to finish?”. This intentionally leaves out the last 1% because those are often outliers.

Wordfence – 99th Percentile Response Time

As you can see above, the un-cached 99th percentile hovers right around 5s when Wordfence is enabled or disabled. Since this is the 99th percentile such a high response time isn’t too surprising in both scenarios.

The more interesting scenario is when caching is enabled. For WordPress with only a caching plugin running, the 99th percentile is 370ms. That means 99% of all requests finished in 370ms. With Wordfence also enabled (Wordfence + caching), that number jumped to 2400ms. That’s a ~7X increase.

Conclusions

From these tests we came to a few conclusions:

Caching does not solve all of your performance problems. If your cache strategy relies on requests making it all the way through to WordPress, then you are very likely to still take a performance hit from other plugins. Things like Cloudflare, Varnish, Litespeed, or Nginx can help alleviate this problem. Running Wordfence is expensive from a performance standpoint. The data suggests that by simply enabling Wordfence you lose about 50% of your maximum capacity and can increase your response time between 2x-7x. Wordfence is still worth it for a lot of people. If you’ve operated a WordPress site for any length of time you know how often they get attacked. Wordfence does a great job of reducing attack surface area and making it hard for people to attack you.

Test the performance implications of your own WordPress plugins with Kernl WordPress Load Testing.