Modern Vespa

hello all, noob here!
recently bought a 2004 ET2 for my lad and have been doing some tinkering, i see you guys on here a pretty clued up so thought i would see what you though of this:
when we bought the bike, the speedo wasnt working (needle kinda stuck/sticking) although the odometer was working. took the speedo display out to see if any obvious issues but couldnt find any. span the speedo up with a square bit in a drill and needle moves up to all speeds and odometer works. put the speedo back on the bike, disconnected the cable from the front hub and span it with the drill from there, again all working good.
took the plastic worm gear out and cleaned the old grease off it and put on some new.
speedo needle now moves but only gets as high as 20mph on a ride (yes we used gps to make sure its not just the bike going slow)
took it all to bits again and pulled the inner cable out to make sure it was well lubed which it was. whilst doing that i noticed that there where some points of resistance along the run, so rerouted the outer cable to allow more fluid bends etc, now no resistance points. took out the worm gear again and cleaned the area its sits in, and scraped old grease from the gear that drives it and put in new. (this just done from within the worm drive hole, i havent taken the hub off yet)
was hopeful that this would help, but still only goes up to 20mph!!! grrrr!
any ideas?

Nobody got any ideas?

In a 20 year old speedo I would suspect a weak magnet. (Were they still using a spinning magnet back then?) I got no idea how to check it.

It may be that spinning the cable up with the drill makes the speedo look ok but turns it way faster than it would on the road, which would suggest the speedo is just sluggish - just a thought.