Somes directly like other commenters touch on. Less likely to saturate CPU quickly. Lower cost to run so can have more headroom.

But also the stuff you tend to do to make it fast makes it more reliable.

Local caches reduce network traffic. Memory is more reliable than network IO so it improves reliability.

Reducing lookup calls to other services (e.g. by supplying context earlier in the dependency chain) makes it faster and more reliable.

Your code will probably branch less and become more predictable too.

And often the code is simpler (sometimes not when a performance hack is used)

First, if a performance optimization is a reliability regression, it was done wrong. A bounds check is removed because something somewhere else is supposed to already guaratee it won't be violated, not just in a vacuum. If the guarantee stands, removing the extra check makes your program faster and there is no reliability regression whatsoever.

And how does performance improve reliability? Well, a more performant service is harder to overwhelm with a flood of requests.

Less OOMs, less timeouts, less noisy neighbors problems affecting other apps