as brutal said the speed doesn't change the compression and personally i don't think the speed has really changed in the way you think it has. the thing is with all the the spark plugs out the only cylinder that has any sort of resistance is the one you are testing so instead of hearing the starting speed change when every cylinder hits its compression stroke you only have one cylinder that that is making compression which if thats the only sound your focusing on will make it sound like its going at like 1/4 speed. and explains why you thought it slowed down on your other car too. in reality it is running a little faster because it has less resistance to fight against.