I found 5th at 45 to be good, with a 140/90 rear tire
Not for power of course but for a nice relaxing engine note
Lugging an engine is bad but you're not lugging a stock S40s engine at 35 in 5th at lite throttle
if you are lugging, you know right away, the bike starts bucking and jumping and making noises that tell you 'I'm not happy'
otoh, there's nothing wrong with buzzing along at 3500-4000 RPM if that's where you're happy, neither is abusing the engine, and it's debatable which is better for fuel economy, as some MC engines actually turn in better miles per at higher RPM ranges
I used to putt around on mine and get like 35 MPG, or 40s at 70 on the highway, but I thing I got 50s at around 55-60 MPH