Could be anything at this point, it's hard to guess if you don't give more details.
Does it accelerate slowly, does it make any noises/vibrations, did you have other problems in the past? Does smoke come out of the exhaust, if yes when and what colour? Does it overheat when youb keep pushing it at 100km/h, did you notice poor fuel economy? Are any warning dashlights on? Does the car roll easily when depressing the clutch/selecting neutral, or does it come to a stop quickly?
It could be a leaking turbo then power will be low at all times, it may also be in limp-home mode, power will sometimes only be restricted by a lowered rpm limit.
If you do suspect the airfilter(i doubt so), take a look at it. Try to drive without it(that's fine for a few km's) and see if your problem disappears. If so, then it's solved and you only have to replace the airfilter.