I remember just adding one letter to the instance type in EMR to switch from Intel to ARM... and saw a 20% speedup as well as additional 20% hourly cost saving, all in all 1/3 cost decrease from a single character.
We saw similar gains going from Intel to Graviton 2 on EMR: 15% to 40% speedups depending on the job. Saw similar gains in EMR serverless switching from x86 to arm as well.
The only issue I've had is actually getting enough on-demand instances in certain regions during peak times.
I can't wait for Graviton 3 to be available on EMR.
Some migrations are easier than others.