Add locks and you add many more causes for the main thread to be blocked beyond pure computation.
And that leads us to the real problem. Chips are not advancing at the rate they should. Additional performance gains come from adding multiple chips to the same die, not making the chips faster. This forces programmers to think about parallelism, which makes everything harder.
In the mid-90s, chips were improving so faster that you wouldn't need to consider special tricks, you could just wait 3 or 4 months and a chip would come out that handled your application.
Now single processor performance is basically at a standstill and it's up to the software developer to spread work in parallel to make things faster.
A weird interpretation of a singular event in the history of technology. No wheel maker complained because wood wasn’t getting twice as strong every 18 months.
We’ve been taught for decades that chips will double in performance every 18 months (I know Moore said transistor count, not performance). It’s now turning out to be untrue. We think about the singularity all the time, but haven’t properly considered what will happen if technology improvements stall.
But good luck with that! You're fighting against the most powerful force in the universe: economics (more specifically sunk cost into existing investments). "Soft"ware is a lot "harder" once economics are taken into account.