The engine and definitions updates can change several things, including how certain types of files can be scanned, which modifies the scanning speed too.
Unfortunately, if there is some speed change, only avast developers can change, correct, improve or answer this question. All we can do in this regard is guess.
Most of us, forum members, have no way to answer this question, unless already answered elsewhere by Avast Team.
Keep paying attention, sporadically, to the scans, and I guess it is probably going to change again (according to changes to the engine).