Hmm that seems excessively slow. I'm assuming you're using multithreaded compiling and SSDs? Any huge libraries? I'd try Build Insights. It seems worth investigating because 800 source files is not really that many.
Shitty computer plus SDD almost full.
I have tried cmake target_precompile_headers, but it doesn't seem to work correctly with some third vendor libraries we are using (precisely the largest ones).
If this is for a business, why not replace the computer? If you can save a lot of time per build it shouldn’t take very long for a new computer or upgraded SSD to pay for itself.