Slipping Clutch

[Fazerider]

That does sound like a slipping clutch.
Check the cable adjustment is correct at both ends, handlebar and the actuating mechanism in the sprocket cover.
The clutch on the Fazer is a bit marginal. When new it’s good enough to transmit peak torque, but the springs gradually weaken with age and then it’ll slip, especially when the bike’s not fully warm and the oil is thick.
So unless you suspect that someone’s done an oil change using car oil with moly and the cable adjustment is correct, it’s probably just the springs that need changing.
Stick to Yamaha ones. Fractionally stiffer springs would be a good idea but for the fact that nobody makes ‘em. You can find heavy duty ones, but they’re massively stiffer and make the bike a nightmare in slow traffic.