This is a major problem with the restrictive Amtrak operations. A lot of tracks get upgrades but time savings are still trivial when lots of the extra speed gets commited to padding. A 14% speed increase should result in at least a 10% time saving.
Acceleration time.
Extrapolating from Fairmount DMU study, there's a .03 mphps decrease in acceleration for a diesel push-pull train for every 5mph of speed starting with the acceleration from 25mph to 30mph. So to accelerate from 80mph to 90mph would take 163 seconds and take 20,458 feet. In the meantime, a 79mph train has covered 18,891.7 for a net gain of only 1,566.8 feet or 13.5 seconds after 3.87 miles of 90mph running. For every ten miles thereafter, you'll gain 50 seconds of time, but you'll probably have stops around that 15 mile mark, if not earlier (unless you're a long distance training running out in the middle of nowhere in which case nobody really cares whether or not you're appreciably faster). So basically you cut a minute from the schedule. It's really not worth all that much.
If you want to speed things up, what you want to do is minimize slow periods and sections (because they also have deceleration penalties) and raise your ability to accelerate quickly so you can spend more time at high speed. Nothing beats electrification for that, especially when combined with the use of distributed traction. Refer back to the Fairmount DMU study. A diesel push-pull train takes 166 seconds and 8,388 feet to accelerate to 60mph. The GPS logged actual journey time and distance of a BR Class 395 doing the same is 32 seconds and 1,498 feet. Assuming no further acceleration and no deceleration, the 395 passes an arbitrary 5 mile marker after 315 seconds while the diesel push pull train does so almost a minute later after 370.7 seconds. The more common your stops or slow zones, the greater the benefit of electrification's higher acceleration and the lower the advantage of a higher running speed (in fact, it would take 8.13 miles and 7.73 minutes for that diesel push-pull to accelerate to 90mph in the first place which makes such an upgrade nearly useless for many intercity runs; by contrast, the 395 does it in 57 seconds and 4,320 feet, before the diesel push-pull has even reached 30mph).