Contention is very common, and it not only affects the ISP's bandwidth but of course the "target" web sites, general internet loadings etc.
Your most recent results don't look too bad TBH, at peak times even 10% of off-peak performance may be "acceptable".
My T**** performance used to drop below dial-up at peak times whereas at other times I could get close to my full 8mbps connection speed. That was a 100:1 ratio or worse and NOT acceptable.