That does sound much too slow. When running right, even stock, they are reasonably stout. Not fast, but easily able to get up to the speed of traffic with minimal drama.
Either a mis-adjusted injection pump or incorrect cam or IP timing, those would be my vote for most likely. Other common causes for "low power" would be restriction in the intake air, exhaust, or fuel supply, OR a turbocharger issue/failure as you mentioned. HOWEVER, with all of those, you would typically see the power loss be even worse at high RPM, when the engine would start to really choke on the lack of air/fuel flow.
In your case, it sounds like the opposite -- very weak at low RPM, but if you can get the revs up enough, it finally starts to pull a little better. Right?
So it's more probable that it is an engine tune issue for yours. Internal IP misadjustment, or incorrect injection pump static timing, or even external misadjustment of the throttle linkage causing the pump not to reach the full "throttle" setting even when the accelerator is floored. You could check last one that easily enough with a helper. The other checks will require a little more in-depth work.
VERY common with these engines is someone attempting to replace a timing belt or R&R the injection pump without the required special tools and methods, and then the timing is off as a result and majorly affects performance. So on your list of things to check, injection timing (using dial indictator) is near the top.
If that is confirmed within spec, then you would proceed to check turbo boost, and then finally to try adjusting the "smoke screw" on the pump and see if you can regain some power. But you only want to mess with those things after confirming correct base engine timing.
If injection timing is found to be wrong then that almost certainly means cam timing is also wrong. At that point you would want to just dig in for the whole timing belt refresh job since you say you plan on doing it anyway, and then continue with further diagnostics after that's done.
Do you ever see smoke from the exhaust? When and what color? How is the cold starting of the engine, easy, difficult, smoky, clean?
Yes they sound wonderful with the exhaust opened up.
