@heciluss, Why it happens can be figured out if you create a slowdown dump and post it here so it can be analyzed to find the cause of the sluggishness. Here's how to capture a slowdown dump:
viewtopic.php?f=12&t=6 (procdump -n 10 -s 1 explorer.exe)
It can be that the location where the Classic Start Menu writes the cached data is not writable on your PC, or its security permissions got messed up or maybe some security app is blocking it. If the menu cannot write the information to a datacache.db file, it will be slow.