But there might also be a performance hit with a GPU... There are various antipatterns like drawing on a canvas and then reading back the pixels repeatedly that perform atrociously with a GPU (due to the latency between the CPU and GPU). Some sites do a lot of that for things like custom font rendering, fingerprinting user hardware, custom data decompression algorithms that use canvas, etc.

Lab analysis is always going to have compromises. What's important is the relative change (without and with the extensions), and that the extra work isn't especially slowed down because of lack of GPU.