Performance of PhotoLab is not really good until the program has indexed a folder completely and created the previews in the cache. When I upgraded from PhotoLab4 to PhotoLab5 I experienced that all previously generated previews were lost and PL started to generate them from scratch again. This is frustrating and really time consuming.
My wish for PhotoLab6 is, that it not only upgrades the database from the previous version but also copies the cache from the previous version.
If you still have PL4 installed, you can copy the PL4 cache to PL5. AFAIK they should be compatible.