it seems I'm having a similar issue to what I experienced playing River- for some reason (Mac? new Love?) jumps on my machine are waaaay smaller than they should be. I can't jump over the first step, unfortunately

As an experienced Mario player I knew I could walk up the game screen and have a shortcut :D One thing I noticed is either players collision from left side or the spikes from right side is shorter so I was able to get passed after the second checkpoint.

I finally completed this game. Great idea of speed-ups of time! Nice game!

But I feel character collision is too large contrary to apparent, and some traps are unreasonable.

One of those games that makes me wanna throw my keyboard out of my window :) and combined with this happy tune.

But seriously, it's interesting game and not  frustrating. Very hard and unforgiving though.

The mechanic of sprinting speeding the obstacles up sounded really cool at first, as it adds choice to something that in a lot of games (e.g. Mario) you would rather constantly be using. However, it doesn't feel very well executed, mostly because I was never sure whether I should speed up or not - you might know as developer, but as a player there was no way to figure it out. Combined with the fact that every challenge seemed to need frame and/or pixel perfect actions and it ended up being more of a game of luck (holding left/right and seeing if I made it through). I think this would work better with more open challenges with combinations of different obstacles - maybe some could slow down when you speed up instead.

what a hard game

Decent game; I like the art style, the blocky, monochrome aesthetic is really nice.
The difficulty ramp needs to be revamped though, making it a bit easier in the beginning.
You should also consider making the hitboxes for the hazards smaller; at the moment, the collision feels a bit unfair, as you're able to die without the player sprite even overlapping the graphics of a given hazard.
I love how speeding up the game increases your momentum, allowing you to traverse larger gaps; it's quite creative, and leads to a lot of interesting level design opportunities.

Waaayyy too hard right out the gate. I'm not given time to learn the mechanic before being presented with a literal wall I have to get past. The second puzzle on the other hand gives you a more controlled environment to see how the speed effects your own movement in a tangible and clear way.

I think the spikes that sink and rise would be more fair if we could see them embedded in the ground, or get some kind of telegraphing that they're about to come back up since the moment they're visible they're lethal.

Graphics look good! You get a lot of mileage out of that monochrome style.  Overall I found the game kind of frustrating but I'm not really that into precision platformers.