I had TS400-K back in '75/76 when working at Suzuki dealer, bike was a tradein that had was absolutely gutless 80 kph/50 mph tops.
Picked it up at our traded price and proceeded to make it run properly. Opened up the air box intake vents, rejetted carby, burnt out the exhaust carbon, decoked the exhaust port etc.
Anyway after a lot a fussing around, mainly jetting, in the end I had a TS that was capable of a genuine 160kph/100mph, had a flat torque curve and was an absolute blast to ride on the street not so good on the trails.
Persist with jetting and make it breathe better.