It is the grapple jump. Once you do that though and go to the next screen, you can't ever go back.
You have both mouse and keyboard controls would be nice to be able to know what the alternative controls might be. And it is a bit confusing when you have the grapple and tethers both on the mouse. Would have been better if the the tether was on the shift key or something for more natural movement. Was jump on space? I forget. Point is, if I take a break and then come back, I might not remember the controls. Combine that with controls that don't necessarily feel natural (I feel like grapple and tether should have been swapped), the player might have a harder time. Giving the player some remap options, like say for me, space to jump, wasd controls, shift to tether, and left click to grapple would feel the most natural because of where my hand rests on the keyboard. This can get especially confusing when I need to jump, grapple, and tether at the same time.
It definitely felt like there was more than just two speeds. It felt like four speeds to me. Might have been a glitch or maybe my senses were just being a bit weird, but I feel like at least once the swipe was super fast. But with RNG, you can get annoying combinations like string on the upper left corner, button on the lower right.
There might not be many, but for a tough fight where you might have to repeat the fight...
The problem isn't a matter of patience. The slug moves faster than you from what I can tell and it can hit you while you are jumping out of the way in some cases. Combined with seeing fireballs and iceballs flying through the boss and it feels unfair.
Still would have been nice if you could keep charging it at least.
There were two flies that I couldn't seem to get right away. The water fly (I don't think you have jump at that point) and the fly on the cliff (due to perspectives, it made it harder to tell where I was even supposed to jump from if it was possible). These two flies really felt like you needed grapple to reach because with the perspectives one, it felt like I kept hitting a wall. The water one just looked like it was intended for later.
That also brings up another possible issue. I wonder if any of the fly traps allowed you to accidentally grapple jump and lock yourself in.