Much of the "lag" problems were tracked down to
1) perception - misunderstanding that the screen orientation switch has an intentional lag. (Irrelevant if you like it or not... that's the way it is.)
2) bad apps - Android is an open platform. Anybody (and their mother) can write an app for it. The "professional" look can now be accomplished by your average 9 year old. There are little to no safeguards that the apps are using the memory areas correctly, removing themselves from memory when turned off or deactivated.
Try to remember when the system started to slow down and removed the apps you loaded from that point. Use a cache cleaner too. You'll probably find that removing one particular apps fixes your issue.
If that doesn't help, it probably means some app scribbled where it shouldn't have. In that case, a factory reset, followed by a careful and deliberate reload of each app you need is called for. Check out the performance of the system after each install before you load the next app.