It depends on what you mean by harder. Generally I'd say yes. However, a slow piece with polyrhythms (even 3 vs 2) for a beginning student who doesn't have the technique will find the slower pieces harder. Very difficult is legato in one hand versus staccato in the other until of course you can do it. Playing large intervals such as tenths and sometimes tenths with a note in the middle seems to me just about impossible at any speed.
I personally have a problem with fast pieces. Scales, Chords, arpeggios, Hanon, Schools of Velocity, have all helped in the learning process of the pieces and of course with muscle memory but this is one area for me I still have trouble with and probably always will. I have even tried the short pulses method to "grow fast muscles" and this did improve my relative speed but I have this habit of constantly raising the bar. I have recently returned to level 1 and level 2 pieces and method books to see just how fast I can play these exercises and pieces (a sort of race with myself trying to play them as fast as I humanly can). They turn into a different animal at speed. The same piece is always more difficult the faster you play it. If there is an exception to this principle I haven't come across it yet.
The 3rd part of Joplin's Maple Leaf Rag with the huge rapid hand shifts in the left hand comes to mind. Over practicing (hands separate of course) helped but this still is the bottleneck of this piece. I have this part up to 44 beats per minute only with a lot of practice and so the whole piece suffers. Practicing take off and landings helped as well.
Mozart's sonata in C (1st movement) has lots of scale like runs in the right hand. Playing evenly is not the problem for me but again speed is as I personally just love the way it sounds performed faster than I currently can play it. However, slow without mistakes is always the way to go.
Burgmuller's The Clear Stream is M.M. = 176 allegro vivace is only level 3 on this website but speed again is the most difficult part. But so what if I can only play at M.M.=160 after lots and lots of practice slowly dialing up the metronome 40,50,60,80,100,120,126,132,138,144,152,160
(It's actually even slower than that). Yes, you can see the improvement and hear the improvement.
The three pieces above all helped my technique immeasurably.
Also, faster pieces will take longer to learn (perfect?) by the metronome approach and should make you consider things like fingerings, and economy of motions. Speed walls occur when you aren't using the optimum technique whatever that may be.
Bach Prelude in C minor BWV 999 has been called "a show off piece" somewhere on this website as it can be played exceedingly fast (just not by me M.M.= 100 is not that fast) but like everything else it's a work in progress.
In summary then, perhaps these "fast pieces" require one to learn techniques they don't currently possess or to build upon what they do.