Inno Setup: Delay Before Progress Bar with Large Compressed Archives

[Masquerade]

It's not because of precomp, it's because of the lack of stdio processing for decompression. Whatever plugin you use to bridge FreeArc with InnoSetup will only record finalbytes written to disk. If you're using $$arcpackedfile$$.tmp / $$arcdatafile$$.tmp for any decompression commands, FreeArc has to write those files to disk, do decompression, THEN write the final files/bytes. This janky process is why you wait forever to see any progress.

You can save all this trouble by using stdin/stdout supported tools for decompression.