One thing I'll note is that, especially if a story is something you've had in your head for a long time, sometimes it will feel "too fast" when it's actually moving at a good pace.
The reason being, when you were imagining the story, you kinda imagined some ambiguous filler space between major events happening... but actually, there was nothing interesting in that space, your brain just always assumed some space would be there. So when you actually sit down to write the story, these moments seem to zoom by, but actually that's the right pace for the story!
But for pacing problems, I think planning helps in both cases. Even if you don't Write A Complete Script, if you sit down and figure out what's happening on each page ahead of time, you can evaluate if things are moving too fast or too slow.
If you find yourself skipping important things or important character moments, or moments that are really significant are kind of getting glossed over, then when you plan ahead, you can see that things are getting missed, and you can look through your plans for moments where you can focus on those significant things that are getting skipped over. Do you need an extra page here to see so-and-so's reaction to this thing, or to introduce some concepts of your world in an early scene so you don't have to explain them all really fast in a big infodump later, etc? If you plan ahead, you can make those changes before you start on the pages themselves
Something I do to see if my comic is moving too slowly is, I write down what's going to happen on each page, and then I write down the names of months next to it. So like, I updated once a week, so if I had:
--
Page 1 - they run from the monster,
Page 2 - they find a cave and run inside
Page 3 - they're still running
Page 4 - they run deeper into the caves while the monster follows,
Page 5 - they reach a dead end and the monster approaches
--
then if I write "OCTOBER" next to Page 1 and "NOVEMBER" next to page 5, then it's easy to see that we've spent the entire month of October doing nothing but running, with no real change to the situation. That's probably too long! So then I might want to cut it down -- Page one they run and find a cave, Page two the monster follows and they hit a dead end -- now there's a New Development in each update.
