The explanation is this (as you also can find it in other topics): BitDefender products have a SmartScan module, which prevents OnDemand and Realtime scans to scan known clean files again on successive scans. This module has a predefined list of files (which is updated along with signature updates), and also a dynamic list (generated on successive scans on your system). So, basically, the more scans you do on your system, the faster the scans should get, as known files will start to be skipped from scan. Don't worry about these files getting infected, as once they are modified in any way, BitDefender will scan them.
So back to your original question: by uninstalling BitDefender, you also removed from your system the known files database. So it's only normal that after a clean install, scans will go slower for a while, but this should get "fixed" in time, after you scan your system a few times.
Cris.
