SMBX-SCREEN SCROLLING LAG

I have a problem in SMBX where in my ice level, with all ice and snow blocks slippery, and an Ice Cap Zone remix, "Ice Cap 4 Life" as the background song, and especially with invisible block puzzles, which are also slippery. I noticed that if I go high enough, then move my mouse, it leaves a trail that stays, plus, if I reach the top, then scroll down, the screen will stay the same for a few seconds, then it will seem as to having the top screen lower. This glitch started before I put the song in, and it was when I made the boundries for a level that scrolled for about 5 seconds high, and 3 seconds long, I shrank it to 3 seconds high as I was making the level. When the glitch first started, I had the snow hill background, but this computer BARELY lags, while the last computer lagged so much. Yet, this glitch has never happened before to me, could you help me please?

P.S.: I have only seen this happen in level editing mode.