Is it though? Would you even notice it?
If there was an already short processing action, e.g. 1 second, now it would be 850ms -- hardly noticable.
If there was some long processing action, say 5 minutes, now it would take 4 minutes and 25 seconds.
Hardly something to write home about in either case...
Look at CPUs or the Internal Combustion Engine. After 120+ years, it still improves, but you're looking at single-digit percentage points if that.
I'm not aware of a lot of things that could magically improve browser performance, but if you do, you have a great opportunity for a unicorn startup.
Today you can buy relatively affordable cars with 2.0 liter engines pushing 300 or more horsepower and doing so with greater fuel efficiency than cars with much more modest output from ten or twenty years ago.
(admittedly some of the gains in efficiency have been obscured by increases in vehicle weight)
We've had export / fx processing improvements in NLE programs or 3D rendering for example that saved weeks over a year. PHP 6 to 7 got several times faster in common use-case workloads. And so on...
It's not like we don't have a yardstick for actually noticeable speed updates. Pre-JIT JS to JIT JS for example was a big several times speedup. PHP 6 vs PHP7 was several times as well (no JIT involved here, just other optimizations).
15% is only 'significant' in the context of an already heavily optimized program (which FF's JIT is). Not significant as in "the users will notice it".
> when Warp landed:
>
> - 20% on Win64 GDocs loadtime: https://mzl.la/3cp6dAs
> - 13% on Android Reddit SpeedIndex: https://mzl.la/2RUWdp8
> - 18% on pdfpaint: https://mzl.la/2HtXb9W
> - 8% on tp6 JS memory: https://mzl.la/3j2VwGb
> - 8% on damp (devtools perf): https://mzl.la/3kLbhSM
Sounds pretty significant to me