Perhaps Your speedometer magnets are weak

consider that the cable doesn’t lose any turns even if it’s dry,
One turn in equals one turn out the other end
If turns build up inside the cable that will be a very short lived situation as the cable will quickly snap clean off

So cable friction can’t slow a speedometer
a dry cable can momentarily stick and cause irrational jumping so lubricating it is a good idea
But to get your missing mphs you will need to service the speedometer
There are a few discussion on here about how to do that
Search the old posts
-s