Performance analysis (Wikipedia article) deals with analyzing how computing resources are used for completing a specified task.
Profiling is one relevant tool.
In microkernel-based systems, there is generally a considerable RPC overhead.
In a multi-server system, it is non-trivial to implement a high-performance I/O System.
When providing POSIX compatibility (and similar interfaces) in an
environemnt that doesn't natively implement these interfaces, there may be a
severe performance degradation. For example, in this fork system
call's case.
Unit testing can be used for tracking performance regressions.