I've never had a 755sa 01xx, so I can't really compare (the "a" version never got to 01xx)
However I did notice the slowdown you describe with an "a" model but I was under the impression its occurrence lessened with later firmware. As for the 760a I do have 00xx and 01xx variants but the 01xx act as spares and I don't really have extensive scans from them to create a valid comparison. I've never seen the slowdown either with 00xx or 01xx variants of the 760a.
AFAIK on the 760 the change from 00xx to 01xx went with a new laser pickup. Apparently the same as used in Liteons LH-20 series. I have no clue that changed on the 755sa from 00xx to 01xx.
One thing to remember is, that the 755 was the original 760 whose introduction was postponed because of quality problems with 18x writing. So the original 760 was relabeled 755 and an overhauled 760 was introduced later. There are some hardware differences between the 755 and 760:
It's only my speculation but maybe these differencies where also responsible for the slowdown you describe? Perhaps some resonance in vibration that required a slowdown?
The 755SA was still produced when the 760A was already available. Maybe later models of the 755SA used the same parts as the 760. Could you look inside and check if the disc-holding table is of the same design as the 760 one?