Indeed in workloads exposed to return trampolines overhead, there can be quite a noticeable performance hit. This includes various network and I/O benchmarks and does translate to performance penalties in such databases (e.g. KeyDB, RocksDB, SQLite, LevelDB) in particular. The WireGuard VPN software for Linux also saw a noticeable hit when the Alder Lake desktop was running with spectre_v2=eibrs,retpoline.
It ultimately comes down though to the software under test for the performance penalty from Retpolines on modern CPUs. Web browser benchmarks on Alder Lake were not impacted nor some of the other tested workloads besides the ones noted in the side-by-side comparison above for where there was a measurable difference.
