If it's not bucking and shaking it's not lugging. Just because an engine sounds like it's providing power while under load doesn't mean it's lugging.
Lugging usually happens at very low RPM, close to idle and when demanding high power. You'll feel it then. It'll buck like a wild horse. I've never seen any car lug at anything over 1,000 RPM. Even my very low powered MGB will accelerate smoothly uphill at a 1,000 RPM or more.