Faster on sunspider. In practice, on real web pages, as someone who occasionally browses the web using a low end tablet PC (Surface Go 2 with linux. The CPU is downright anemic but good enough for treating it like a reading device/video stream platform), I experienced many more stutters on Firefox that I did not on Chrome and it has become one of the reasons I stopped using it (the syncing is also way worse and often breaks. "Read latest tab from other device" is almost useless if you don't manually trigger a sync because it sure won't do it by itself in any reasonable time frame).
The performance difference is much less visible on my main computer though. But yall really need to try firefox on low end devices before you make generic statements about its performance. Or the android version.. lord, firefox on android is just unpleasant.