The only thing that I can see that might (not does) explain why two switches are order of magnitude slower than one is that you are using the same output pin for all of them. You might try changing to use 4 different output pins. Nothing in the code suggests why this might be a problem.
One other thing to try is to see if it is one particular pin that is causing a big delay. If each pin by itself is fast, then I don't know why two or more are slow, unless using the same output pin is a problem.
Maybe...
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