I think 5% is a reasonable guess for a database. Even a well-designed database does have to do a lot of IO, both network and disk. It's just not a "fixable" thing.
But overall, yeah.
But overall, yeah.
The reality is that OLTP databases execution time is not dominated by CPU computation but instead of IO time. Most transactions in OLTP systems fetch a handful of tuples. Most time is dedicated to fetching the tuples (and maybe indices) from disk and then sending them over network.
New disk devices lowered the latency significantly while syscall time has barely gotten better.
So in OLTP databases I expect the impact to be closer to 10% to 15%. So up to 3x over the base case.