I've had my TD 14 years, and, yes it was sluggish compared to the 3.2, however, now it is chipped and with an AR35 box, it has put a smile on my face.
Simple things first:
1. PROVIDED that your coolant system is in good shape, clean, flushed, and filled with GM red antifreeze 50/50, remove the viscous fan, if fitted.
2. Check the rubber (possibly braided) pipe from the centre bottom of the air intake manifold to the electrical gizmo (an expert will remind me what this does

) next to the power steering reservoir. This pipe needs to be clear and have no splits anywhere.
3. If service history not known, or in any case, give it a thorough service, oil change, GM 10W-40, oil filter, air filter, and fuel filter. Before connecting the latter, fill the housing with Diesel Magic (about £6.50).
Experience by OOF TD owners has demonstrated that oil changes should be done every 3000 miles on this engine.
Report back if it is still down on power.