Xerces is an old library, probably ported from autotools and will do a lot of autotools like checks which for CMake means: creating a CMake project with the probe, generating the build files from it, building it, gathering results. Once for each test. It's SLOOOOW.

I recently (a few weeks ago) made a project to batch those type of commands together and parallelize tests as much as possible. You can check a merge request for CMake upstream here: https://gitlab.kitware.com/cmake/cmake/-/merge_requests/1226... and the associated issue for more details.

In general, CMake is fast enough, the only slow bit on the critical path is detecting the toolchain. The rest is quick, but if your build scripts are using slow serialized functionality, you'll be paying that price. But only once as the slow results are cached, so next incremental run should be pretty fast!