I was updating from 4.0 to 4.4 And went with latest version of 4.40.2 and was having lots of issues with it. Was slow and unresponsive and was bring site down and back up again.
Don't know what the 4.40.0 or 4.40.1 was like But had to update to 4.40.3 and now all seem good.
Release of 4.40.3 had these notes, so guessing on of them was issue #5586 Fixed the issue with increased memory (moved back to Autofac) #5587 Added caching to the "GetAppliedDiscounts" method #5592 Routing errors when English is disabled #5591 PayPal SmartPaymentButtons plugin. No redirection to the confirm page #5590 Null validation when searching #5561 #5545 Error when filtering on the order list page
There was an issue with 4.40.2 consuming too much memory, but that has been resolved with #5586, and also #5587 may help - I haven't tested 4.40.3, but my basic install of 4.40.2 doesn't have any issues.
I guess it depends on your server specs (ours is rather high!): http://nop440.socksandboots.com/