Quality of the track plays a big part of course. I have never experienced such a rough ride as I did on my way back from Bodmin in a HST▸ a couple of years ago. It’s not often that train travel makes me anxious!
Clearly the quality of ride is (a) dependent on track, (b) speed and (c) in the absence of accelerometers the judgement is subjective...
...BUT the comparison with HSTs is still valid. As far as I am concerned the
IETs▸ are not such a step forward from the HSTs - not as they are now, but as they were when newer - as the HSTs were from the Mark 2s and Mark 1s they replaced. There was a silky softness to the ride of the Mark 3s and the traditional jerk when the couplings tightened as a loco took the load vanished.
Over the years they deteriorated due largely to maintenance which did not restore parts to their original condition - this was exceptionally bad towards the latter days of
BR▸ . One of the first things that Great Western Trains did was to have a campaign replacement of the rubber bushes at each end of the longitudinal traction bars on the outside of the Mark 3's bogies. This removed much of the uncontrolled motion and bumps which had become very noticeable. A later change to the wheel slide controls reduced the incidence of 'flats'.
A change which made the Mk 3 more susceptible to track imperfections was the need to modify the swing link length to avoid possible contact with the third rail when running on the Southern. On good track this change was not really noticeable, but on 'iffy' track the lateral motion became 'nervous'.
It is entirely possible that the freshly maintained BR(WR) track was kept to tighter tolerances in those days with a much lower traffic density than is possible now, but one would expect a modern train and suspension to be able to cope with todays tolerances and STILL give a better ride.
This is not the case. I came back from London to Reading on the 21.15 IET to Swansea a couple of weeks ago and timed the run using my trusty iPhone. We left Paddington on time and passed Slough in 11mins 49 secs., Maidenhead in 14 mins 40 secs., Twyford in 17 mins 59 secs. and stopped at Platform 9 in Reading in 22 mins 17 secs after quite a slow approach.
The performance was what one would expect from a modern train with a sufficient power-to-weight ratio. However the ride was appalling - there were quite high amplitude high frequency jiggles both vertically and laterally above about 110/115mph and some loud clonks from underneath especially over pointwork east of Hayes. The ride seemed a bit better further west but I think the driver eased off after Slough.
In a brand new train this is, frankly, unacceptable.
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