I have a alpha carette compresor that used to fill my tank from 3000 psi to 4200 in about 2 hrs now it takes all day. And this pump only has 55 hrs on it.
In my experience with high pressure compressors, it’s the valves in one or more stages. But first definitely check for leaks in any external connections with soapy water or a commercial liquid leak detector.
Yours just might need cleaned up. But if it needs parts PM Bosun and he can get you parts. I had to rebuild my alpha carette not to long ago. Shipping was suprising fast.