The #1 thing that causes delays when Mozilla or Firefox windows are brought to the front/unminimized is that Windows likes to swap its memory to disk ASAP.
It does this to Mozilla and Firefox more so than any other applications it seems, and I strongly suspect this is intentional. If you are on a laptop it will magnify this effect because laptop hard disks are painfully slow.
Your best bet is to make sure your swap file is not fragmented.
If you are using the stockticker extension, this can have similar effects, even more severe when opening a new Firefox window or tab.