Glad you got it going. The slowing down for me seemed to be more related to the map rather than the model of tomtom. It starts slowing after constant use in a moving vehicle.
Leave it on in the house overnight and it appears to be fine. I now trust my 820 with camper maps with a 15'7 trailer into town centres. It doesn't do stupid routing like
the new go professional range, seems to be the same as an old go 520 with 9.702 navcore.