Most phones have an A and B slot partitions. This makes OTA updates possible. Say your phone is running on A slot boot/recovery/rom partitions, when you do an update it writes the new firmware to the B slot, once it’s determined that it was successful it reboots the phone and swaps the active slot to B and boots up. Next update would write to slot A. You can check the active slot by doing

fastboot getvar current-slot

Then swap the active slot by doing

fastboot set_active a

(or b, whichever isn’t active)

Have you tried swapping back to the inactive slot? Then maybe start over again from scratch.

Seems pointless. In most mobile devices speed isn’t the problem, heat is. Phones, tablets, and even most laptops get thermal throttled within seconds of running at max CPU.