Many journeys are now longer than they were years ago, there was a big news article about 8 years ago on the subject. It seems its mainly (in the London/southeastern commuter areas anyway) down to modern safety standards, increased traffic and lack of slam doors.
The slam door seems to play a big part-years ago most people would have got off the train and many would start getting on before the train had actually stopped. Now the train has to stop (which with modern, almost over cautious, driving techniques preached by management takes longer than in the slam door days where drivers were encouraged to hit platforms much faster than now), the doors must be released, opened, people get in and out, once clear close doors, safety check, start moving takes much longer. I seem to remember the report in the press blamed the lack of slam doors as the biggest factor in slowing down journeys.
Safety standards have certainly slowed things down, companies over-enthusiastic driving policies (many of which I don't agree with-the principle of many is ok but the fact we can't use common sense seems stupid-having to take only half power on a yellow even though the red is on or past the end of the next booked station platform etc).
I know this doesn't specifically relate to east coast as their stock hasn't changed but it will have an effect with regional traffic which intercity trains are timed around.