[Philip]

I think it is most likely a coincidence that the decrease in performance has occurred at the same time as experimenting with swapping about the wheels on your PT. I doubt that temporarily fitting larger, heavier off-road wheels has caused any damage to your PT. Off-road wheels do increase drag (and thus performance), especially so at lower tyre pressures.

I suspect one (or less likely, both) of your battery packs is getting weak with age, and this has just begun to result in noticeably decreased performance. Your packs are, presumably, manufactured about a dozen years ago?

Or could the decrease simply be put down to a change in temperature (for example, has it got noticeably colder recently)? Or are you carrying more payload?

If you can rule out battery performance, ask a Segway Dealer to inspect your elastomers and couplings (between the gearbox and motor). Lastly, yes a "drunk" feeling could be an issue inside the PowerBase platform (for example, the BSA).