Hello everyone!
I'm working on a little patch for KWin to adjust the global animation speed better, so I'd like to just recompile KWin instead of waiting for the whole tarball to recompile. This would save me a lot of time. Is this possible?

It certainly is - assuming you are performing a local installation.
However, be aware that KWin is dependent on other parts of kde-workspace (such as those in libs/) so they will automatically be built as well.

To do this, run cmake at the top level of kde-workspace as usual, then change into the kwin/ subdirectory which cmake will create and run make there. That will result in only KWin and it's dependent components in kde-workspace being compiled.