Weights on the locomotive axles as I understand. This leads to restrictions on where the loco (and stock) can go, and at what speed it can progress.
For example on the Fenland lines around East Anglia there are large speed differences between Sprinters / Sprinter equivalents (755,s 170s etc) and everything else. Sprinters are allowed to do 75, whereas for locos etc it is around 45 mph?
@dk1 being a local on those lines would know better than me.
Naturally you're not keeping to time if you can't keep to the required speeds.
As regards a "desperate TOC" - can't see it without motive power, and it's rather unlikely DFT are going to let a TOC fund a fleet of locos to run the Mk5 sets around to say the least! So much for LHCS being an improvement.