200k is mild, 450 is old, 600 is end of life
First thing, Check timing and don’t start the car until you have. If your off, your timing belt is ready to snap or bad things.
There are plenty of things that could cause this low power so we will start with the basics of an engine.
The most critical is the timing belt is its fairly easy to check physical timing. Basically you rotate the engine by hand with a socket wrench at the crank, NOT anywhere else, until you line up the 3 timing marks, one on the cam, the lock pin on the IP and the one on the flywheel.
What AGE (how many years since) is your timing belt and how many miles has it been, most owners change it within the right mileage but don’t within the right time frame. You really only have 5 years for a belt to last or your service mileage.
2nd on the list is the most common issues like Brady rogers mentioned. Clogged intake, MAF, vacuum lines, clogged turbo vanes, actuators, vacuum pump; boost leaks and so on for a hefty list of simple issues that are easy fix.
A few words of advice, if your intake is clogged up BAD, you need to clean it AND the intake ports otherwise you risk a chunk falling off into the engine and causing a holy mess of bent parts.
I have to ask, when your oil changes are done, and when was the last one and what oil level is it at now.
If your turbo vanes are getting clogged up, it could be from a worn out lifter or lifters.
VCDS is going to be a MUST for you now, and a VAG-COM cable
You need to check for compression as well. Get a compression tester and a glow plug adapter and carefully take the glow plugs out, if they fight you, don’t bother, best not to snap them off.
Are there any other issues you have, like oil leaks, coolant loss, how often is it serviced and so forth. A bit of history can help.
