Performance regressions are different from ABI incompatibilities. If the kernel refused to do any work that slowed down any userspace program, the pace would go a lot slower.

Or be a lot uglier. See: Microsoft replacing its own API surfaces with binary-compatible representations to workaround companies like Adobe adding perf improvements like bypassing the kernel-provided kernel object constructors because it saved them a few cycles to just hard-code the objects they wanted and memcpy them into existence.