My experience from Thumb was that it makes code slower. Probably because the CPU had to execute more insteuctions than in "ARM" mode. Thumb was a thing for emb

My experience from Thumb was that it makes code slower. Probably because the CPU had to execute more insteuctions than in "ARM" mode. Thumb was a thing for embedded systems with very limited RAM etc. It was not designed for optimal speed. My guess is that ARM64 targeted higher end, and that severe memory constraints were no longer considered a big issue. ARM has its Cortex M4 (Thumb only microcontrollers) and the likes for those markets.