Haven't got time for a full answer at the moment, but yes, slow-downs are probably congestion.
But those photos show your internal wiring is a complete mess and needs serious attention. Nothing that you can't handle yourself if we are gentle with you and you are gentle with one or two wires.
The fact that ext 2 and 3 are not in use doesn't stop their wiring screwing you up, which they are contributing to.
Amongst other things, both seem to be masters. So that's at least three you have. The wiring of ext 3 looks particularly suspect but that's where I need to look closer.
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