As I type I'm doing my 2nd "Service" scenario from Paddington to Reading on a HST, both of which have been given 27 minutes between leaving Paddington and arriving at Reading (perfectly fair and realistic based on real life timings). However, when doing the scenarios, sticking to the speeds (i.e. 40mph, 50mph, 100mph then 125mph out of Paddington), time just disappears. On the first one I completed I ended up 8 minutes late for absolutely no reason. And the one I'm currently doing (1C01 Paddington to Swansea), it's 0547 and I'm still 9 miles away from Reading. Has anybody else noticed similar patterns?
It's a common issue, another thread about it here - https://forums.dovetailgames.com/threads/timetable-issues-and-lack-of-schedule.3816/
You are correct, the HST does not accelerate fast. At least not when you have two seventy tonne power cars with 8 coaches between them, That is a lot of weight to pull when you are in the leading power car.
The HST accelerates accurately and in line with the real life version. If you play this on Xbox One X or watch Xbox One X streams, you will see that the timetable timings are easily maintained!