There is a newer SoC revision on the newer rpi's. For example the rpi400 comes at 1.8Ghz base.

Even so, it seems the 1.5Ghz is more about running the system without a heatsink. Give one a heatsink and they run at over 2Ghz. The perf bump from 1.5 to 2.2Ghz is not insignificant.

https://www.jeffgeerling.com/blog/2020/raspberry-pi-400-can-...

https://hothardware.com/reviews/hot-clocked-pi-raspberry-pi-...

Also: Arm itself says the A55 is ~18% faster than the a53. So, its still quite slow.