Generally a diesel engine needs to be run hard and they are governed to avoid over-revving so are safe to run fast
If the engine is not pushed then the carbon from burnt fuel clogs up injectors, EGR, DPF, inlet manifolds and this reduces performance and costs lots in repairs
If the engine is run at low revs then pushed hard in a high gear then this means the engine can usually deliver the power by way of the turbo but also causes the Dual Mass Flywheel (DMF) and clutch to slip, also costing money
The trick is to keep the revs high in the appropriate gear until you change up and if cruising on the motorway then fine to change up and lower the revs but at any opportunity to give it a clear out (slip roads are a favourite)