This is a little tip passed onto me by a friend at work.
By default it appears that routers in GNS3 like to use as much resources as possible, shown here.

You can see that Core4 Is currently being totally maxed out. This is with one router, no config running. This is also on a 2.67GHz Core

But its pretty painless and easy to get that usage right down. Just right click on the router, then left click on IDLE PC. Give it a minute to do its calculations, then in the drop down menu, select the option with ‘*’ next to it.

I should mention, you may need to do this a few times to bring it right down.

I tend to have htop running in the background just to certain iv got it sorted.

In the end it will look like this

Job Done