Apple's new 15-inch MacBook Pro can be upgraded to include a 6-core 2.9GHz Intel Core i9 processor that has demonstrated impressive performance, but one YouTuber is warning customers away from purchasing it with claims that the MacBook Pro chassis can't provide sufficient cooling for it to run at full speed.

Dave Lee this afternoon shared a new video on the Core i9 MacBook Pro he purchased, and according to his testing, the new machine is unable to maintain even its base clock speed after just a short time doing processor intensive work like video editing.

"This CPU is an unlocked, overclockable chip but all of that CPU potential is wasted inside this chassis -- or more so the thermal solution that's inside here," says Lee.

He goes on to share some Premiere Pro render times that suggest the new 2018 MacBook Pro with Core i9 chip underperforms compared to a 2017 model with a Core i7 chip. It took 39 minutes for the 2018 MacBook Pro to render a video that the older model was able to render in 35 minutes. Premiere Pro is not well-optimized for macOS, but the difference between the two MacBook Pro models is notable.

Lee ran the same test again with the 2018 MacBook Pro in the freezer, and in cooler temperatures, the i9 chip was able to offer outstanding performance, cutting that render time down to 27 minutes and beating out the 2017 MacBook Pro.

As Lee points out, thermal throttling is in no way unusual and it's seen in all manner of laptops and mobile devices from a range of manufacturers, but he says that "this degree" of thermal throttling is "unacceptable."

This kind of thermal throttling really affects the end user. It doesn't matter what you're using it for, like if you're a Final Cut user, or an Adobe Premiere user, or if you're using it for software development or calculations like fluid dynamics -- it doesn't matter what you're doing with your device. If you have any kind of extended computational work that uses the CPU -- that's probably why you're looking at these devices in the first place -- it's going to throttle. And that's unacceptable to me.

It's not clear if there's something wrong with the MacBook Pro with Core i9 chip that Lee received, because this kind of throttling is likely something Apple would have tested for and not something that other users have reported at this point.

Because this is just one data point, it's not enough information to reach a conclusion about the i9 chip available for the 15-inch MacBook Pro, but additional testing will certainly follow to shed more light on Lee's video. We have asked Apple for comment on Lee's findings, and will update this post if we hear back.

Update: Other reports of excessive i9 throttling have been trickling in from Reddit users who have purchased 15-inch MacBook Pros with the high-end chip. These threads are available here and here.