This is a problem I've had to directly deal with twice.

The first time was when I was making platform tests. Originally, I wanted to make 40 tests (with an option of starting at level 1 or 21) all in the same j2l. This proved impossible because of the number of sprites that needed to be loaded at once. Once the level reached about 30 tests, the platforms started disappearing. So, my only solution was to split the test in half: thus, pt1.j2l has levels 1-20, and pt2.j2l has levels 21-40, and the HOFs in both levels show who has beaten both halves. This is an unideal solution, but necessary. In fact, before JJ2+, pt1.j2l tended to have problems anyway (the platforms would usually appear, but player sprites would often have missing frames).

Since you are making a test level, you can try this solution (if you can't reduce the number of objects). Even though I wanted platform tests to be one big level, it still works as 2 levels (the HOF linking helps).

The other time I've dealt with this problem is less relevant, but I'll explain it anyway.

It was while I was making devres ep5. In the level "The Fortress of Decay" I wound up using a lot of collapsing scenery events. Problem is, when you hit these, they persist even when they move far off the screen (unlike most events in SP)... and too many will cause stuff to disappear (including bullets). I had 2 ways of solving this: either I could split the level into about 3 parts (unideal, for a number of reasons), or I could reset the level at certain points by forcing the player to die (note that you can do this without making the player lose a life). I chose the latter option. That's why the level has those weird "decontamination room" things at the checkpoints. They basically reset all those collapsing scenery events, by killing you! A bit extreme for mere decontamination, but as the text string says, "this might sting a little".