The CPU limit has been inplemented because some users complained that the update started in the middle of an intensive CPU task (game) and disrupted their gaming experience. This makes some sense - as the automatic updates may start at any time, and yes, they might be a computationally intensive process if an update really takes place, it's hard to avoid.
However, if the user invokes the update manually, he is hardly just in the middle of a fullscreen game - and he's probably got a reason for the update (wants to start an on-demand scan?) So, the update is performed in the usual way.
I mean, how many programs have you seen limit their CPU usage when you start some longish operation? None? It's normal for a program to perform whatever it's asked to in the usual way, using as much CPU as needed (there's even no documented way to limit one's CPU usage, as far as I know - it's kind of a trick here). It's up to the operating system to distribute the CPU usage amongst concurrent programs, if needed.
So, I must say I'm sorry, but I don't understandat all what do you mean... Yes, avast! is just another application, so it does exactly the same as other applications do. If you ask it to perform some action, it does perform it - in the usual way. Why should it prolong the operation 3 times artificially?
Besides, all this doesn't explain the original post - on contrary, actually. The more CPU is used, the faster the manual updates should be, right?