I was working on a ray-based project and was trying to figure out why my rays were only going partially around the stage and found my repeat block was going all at once. I recreated the script below

when I receive [ v]
wait (1) secs
repeat (12)
repeat ((300) - (Clones))
wait (0) secs
create clone of [ myselfv]
turn cw (0.1) degrees
end
if <<[clones] = [300]> or <[clones] > [300]> then
wait until <[clones] < [300]>
end
end

No, but you can do a run without screen refresh custom block to make it instant.

Yes, if a loop only runs non-screen refreshing blocks (e.g set variable blocks; Motion etc. blocks refresh the screen), and no other screen-refreshing blocks have ran in any other script, the loop will run as fast as possible. Though you do have a turn block and wait blocks in the script so idk.