On a freshly installed PC, I've installed MSE and deactivated all background tasks and realtime protection options. (only to use it as an on-demand scanner)
Still, the PC boots over 10 seconds slower than before the installation. What gives? I've deactivated all the crap. OK, the computer isn't the newest (an Core 2 Duo), this isn't an excuse though.
The speed penaly goes only away when you uninstall it. Do the test too, and you will see.
Avira doesn't slow the PC down if you deactivate realtime scan and all the other junk.
Haven't tested the other bloatmonsters on this computer yet.