Low engine power, low gas mileage

In any gear my car wont rev up past 4000rpm, in neutral it revs up to the red line just fine. Acceleration is extremely sluggish, on a level road it takes a lot longer than it usually did to hit 60mph, on any sort of an incline it struggles to hit 30mph. Fuel cosumption has increased from 5.5 l/100km to 7.8 l/100km.
Theres no check engine light at all. I have cleaned MAF sensor and entire throttle assembly, replaced spark plugs, oil and air filter, had fuel pump and fuel pressure checked. Had cylinder pressure tested by mechanic who told me it is fine. Exhaust system is not clogged, piston rings are still in good condition. Installed a new clutch kit. Had all sensors tested by an auto electrician who saus they are fine. None of these attempts have fixed the problem at all and it seems to be getting worse.
I have noticed it take a while for oil pressure light to turn off upon start up, but topping oil by less than 1 piny seems to have stopped that. Any advice on what the problem might be?