There is an interesting article about performance of sharepoint web server.
http://technet.microsoft.com/en-us/library/cc298550.aspx
It says
Do not leave pages checked out
If you are using Enterprise Content Management, do not leave pages checked out. Instead, if it is possible, check them in quickly after every change. Leaving pages checked out reduces the performance of page rendering.
Well the reason behind saying so is not known...i guess it is just a kind of "Not to do things". I haven't actually tried it.
Hope it helps.