The XC90 brakes are very good - but they have one weak spot - the amount of sub-standard pattern parts available on the market.
They do require a little more effort than some other Volvo models but it's the biggest and heaviest car they make.
I am certain that if you get your brakes checked over and at least the front discs and pads replaced with genuine Volvo parts - you will be happy with them. Also get the fluid replaced if this hasn't been done in 2 years or more - it can make a significant difference on its own.
Volvo still cast their own discs for their bigger XC models at their own foundry in Sweden so genuine parts is the only way to ensure they are going to work as Volvo intended them to. There are a number of dealers selling genuine parts on e-bay at trade prices.
We frequently see new members on here with brake issues on XC90's and nearly always the cause is cheap, pattern parts.
I've just changed the front discs and pads on my own XC60 from "Apec" brand (fitted to the car by the dealer I bought it from) to genuine parts and the difference is night and day - less effort, much more progressing braking and better judgement and feel. It brakes like a new car again, not an old banger.
There is a possibility that one of the sliders has siezed but this isn't common.
A full inspection is needed, not just "through the wheel".
https://www.youtube.com/watch?v=PHD5dWKeRXg