Newbie advice: avoiding beach ball after every action

I am using the FC trial version, and this is the first thing I'm making. So both my terminology and my process are probably wrong.

I have 43 short film clips that i want to show simultaneously in such a fashion that the clips slide from the middle of the screen to their own location in a 8 x 5 (-ish) grid.

Each clip has been cropped to a square format and transformed. The transformation scales the clips to 15% size, and assigns a location for the clip in the "grid". All clips are preceded by the Slide effect that lasts about 2 seconds. After the completed Slide, I need to show 5-10 seconds of the clips.

To get to this, I:

- imported each clip "on top of" each other

- selected all clips and applied a Crop on them

- selected each clip individually and set the Transformation parameters

- added one Slide effect before one clip, set its parameters, and copied it to the beginning of the other clips

I noticed that if I have more than 5 clips active (as in I press V on a clip to toggle it on/off), I get the beach ball. So I did what I described above by toggling the clips so that I had about 5 active clips at any given moment when cropping/transforming.

OK, so now I have them set up as described above, and I want to render them to one clip, but I'm not doing too well. Most of my clips are now deactivated, and when I activate them (one at a time or all simutaneously), I get a beach ball that doesn't go away. So this seems like Mission Impossible.

So is this thing that I'm doing simply too much for my MacBook Pro 2015? Am I doing this the wrong way? I have disabled background rendering, but otherwise the settings are defaults.

As a workaround, I have thought I could make the first frame of the clips to an image file, and apply the Slide effect to images as opposed to the movie clips, but that gives the wrong end result. Another approach might be to render just 5 clips at a time, and then mix the resulting clips together two at a time using green screen or some unknown-to-me mixing method.

So, as you can see, I'm completely lost, and any and all tips are greatly appreciated! Also, I need to get this done...

Cheers,

Kim Johnsson