Northern have, for some years, been under the mistaken impression that very gentle starts with 769s (and 319s before that) are necessary to protect the traction motors from damage. The drivers are instructed to drive in this manner. This despite technical advice from engineers familiar with the trains over many years, myself included.
The reason your train took 1 hour 25 minutes to reach Manchester Victoria (RTT schedule link:
https://www.realtimetrains.co.uk/service/gb-nr:P28810/2026-08-03/detailed#allox_id=0) is that it calls at all gateposts en route (excluding Clifton), and indeed it arrived at Man Vic only as minute late after being up to 5 minutes late en route.
769s in diesel mode tend to be a tad slower overall than the 150s they usually replace, but when in AC mode they romp away as would be expected; performance over the very hilly section between Wigan/Ince and Bolton being much better in AC mode than it used to be in diesel mode, and better than a 150.
Which carriage did you travel in? I can't think of a reason for the/an engine shuddering when arriving at stations. Were you in the Auxiliary Trailer (the non-driving car that doesn't have the pantograph on it)? The main compressor, which is under that vehicle and cuts in and out as required, can sound quite noisy and rattly.