Shader compiler has this problem, because it’s set to low CPU priority.
I made a simple test with a BAT file that temporally “solves” this slowdown:
wmic process where name="ShaderCompileWorker.exe" CALL setpriority "above normal"
If you launch this BAT when shaders are compiling it will significantly speed it up.
I think it only affects older CPUs. Unreal tends to be designed and tested for modern workstations with cutting edge CPUs and a ton of RAM, so bugs like this remain unfixed.