It may be faster than Chrome on Android, but it's certainly not faster than Firefox on Android with adblockers. Not loading/parsing multi-megabytes of Javascript and images is huge. Loads of ads these days incorporate entire JS framework toolchains resulting in a binary bigger than the page they're being injected into. Eliminating a few ads and the rest of your browser could be a lot less efficient and still perform well. That's all before discussing how adblockers also reduce who is tracking you everywhere.
In my experience and limited testing, even with warmed-up caches, Chrome (without Adblocker) was faster than Firefox (with or without Adblocker), sadly.
I still use Firefox due to the security benefits of ad blocking, but I don't find the experience particularly enjoyable (although, thinking of it, it may have either significantly improved in the past ~half a year, or I just don't notice because of a more powerful CPU in my new phone).
Eh. That may be the intent but they aren't there yet. I tested Firefox Preview on Speedometer 2.0 and got the same result I did with Firefox for Android 68. Chrome gets double the score (runs per minute) on my device.