This is pretty mature technology, and like most mature technologies, there aren't going to be a lot of 50% improvements any more.
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.