Does Different RAM Cause Slow Boot Time

[Postal2]

... When I got to his house, he gave me two different manufactures because he couldn't find a matching pair. I wasn't excited about it, but figured what's the worst that can happen?

Check with AIDA64, it can be really slow. Some people just don't realize how slow old hardware can be.

... Now seeing the one with different sticks is booting slower (which isn't an issue), I'm questioning whether the scope won't perform correctly (glitches during measurements, causes questionable waveforms, etc...).

I wouldn't wait for failures and just replace the memory if it's too slow. It can be incredibly slow.