You have to exercise extreme care here otherwise you could exclude *.exe globally and you would be extremely vulnerable.
See image for an example of my exclusions, I have used a wild card *.pqi (2GB disk images) but I have used it within a folder. So the same is more important with something like *.exe you have to do more to limit the wild card, e.g. use it within a folder.
This will not however, speed up your system as exclusions aren't considered by the resident scanner. So if you open (loading/runing the program, etc.) the exe that is excluded from the on-demand scan, it will still be scanned by the resident scanner, as is also the case for scanning created/modified exe files. The time saving on this is minimal anyway.
You could have avast ignore a specific program, so that avast doesn't scan it actions, rather than scan its exe file. This may speed up processing.
Check out this thread for lots of options you can set through the avast.ini file.
Understanding avast4.ini file