It's much faster - not metro train fast of course, but usually one of the first noticeable signs the train is being pulled by an HHP-8 is a quick speed up after passing Ivy City.
*HOWEVER*, I am convinced they are driving them more gingerly than they used to. Nowadays they will sometimes only go 110 on many segments where they could go 125, except for a few stretches like Aberdeen-Edgewood. They don't accelerate them as aggressively as they used to but it depends on the engineer of course, and I guess whatever other reasons. (dispatcher telling them to hurry up?) It's sometimes harder to tell now what is pulling the train just based on first acceleration. Once or twice I've even been sure it was an MPI until getting past Seabrook or Bowie State and then they will start movin'.
I've finally been in a couple trains pulled or pushed by an SC-44, and I think it does have better acceleration than the MPIs, but not as fast as HHP-8 when they are 'driven hard'. Again the issue being, they don't push the HHP-8s like they used to so it's harder to have a baseline for what that feels like. (Once in a while an engineer will really gun it) For some reason although they are finally being used some on the Penn line, it's not for trains going north of Baltimore.
Speaking of "MARC sensations", I think I mentioned this in a post years ago, but the old BWI express that left at 5:20 (train changed in 2013?) used to sometimes take the 80 mph crossover north of New Carrollton, at 80mph. So, you leave DC, poke along to Ivy City, suddenly the train speeds up to 125 - you'd be going that fast well before the Anacostia bridge...then a sudden slow down, slightly uncomfortable or disorienting track change...people would turn their heads up from their phones...and back to 125 until BWI, which it reached in only 25 minutes IIRC. I really miss that train. For some reason the current "expresses" get mired in track congestion more often, but maybe that will change when all the track work is done. (will it ever be? seems to have gone on for years now!) No MARC train I've been on since that one, takes any "high speed" (by American LOL standards) crossover during revenue operation.
[update, just remembered depending on which track it's on at Odenton, it had to take a slow, standard crossover north of there, to get back on the BWI platform track. Point being the BWI express felt fast in a way, today's afternoon 'expresses' often don't]