Note: Microsoft Security Essentials provides for not only exceptions for files/folders, but also for processes. And unlike Avast, it does does not slow down your computer.
Maybe that is because Microsoft Security Essentials is not doing anything?
If you have your antivirus set to not check the veracity and validity of files being called and run, then of course it will not slow your system down.
You might just as well do without in that case.
Seems to be a case of not being able to strip the functions of an antivirus down to the bare bones; Avast! is not set up to enable that function, as it has protective modules built in that will hinder or prevent that.
Is that what you consider to be a (nasty) bug?
Is the bug there because Avast! will protect itself when Microsoft Security Essentials will not? See pages 5-8 of the pdf link Polonus kindly provided. You will see that MSE was compromised the most often of the antivirus programs tested; in some cases it was rendered completely inoperative.
A more important criteria of antivirus operation, one that applies to your point of view, is does it interfere with the proper operation of legitimate programs? On this one criteria, MSE does very well. MSE produces very few false positives, but then again, does it detect enough malicious malware compared to the other antiviruses tested?
Is it protection you demand, or is it seamless operation?
So, which is it?