Car loses power accelerating and uphill?

I bought the car a month ago. I changed the oil filter, spark plugs and air filter, but couldn't get the fuel filter off.

The car doesn't smoke. It doesn't leak oil, although I haven't checked again if it uses oil.

It starts fine. Reverses fine and accelerates well for the most part. It struggles terribly uphill if I don't have forward momentum.

It sometimes struggles with power in the lower gears when accelerating as well.

What could the cause be for the lack of power?