Just for comparison, I'm almost 120lb. 0-55mph in 3 gears, per the speedo. I can go uphill at 45mph in 4th.
I'd say there's something wrong. Being heavier should only reduce the acceleration, not top speed of any gear (other factors aside like headwind, uphill), at least for the first 3 or 4.
I have a hard time believing it's been re-geared that low, but check the sprockets for the correct tooth count, 14/33 I believe. And of course your chain tension while you are there.
Maybe the clutch is slipping, that's what I would check next. If you're lucky the PO just adjusted the clutch free-play too tight. If not you may have to inspect it.
1974 CB125S (purchased 1983, ate 2 crankshafts, sold 1984)
1986 Rebel 250 / N. California (purchased 2019)