Is that only for loading in the beginning? I also notice a bit of delay while starting TheSims4 initially. I know I have plenty of saved lots and multiple copies(with differing names) of ongoing/unfinished building projects.
Earlier in the game, I had already tried managing my tray folder by only liking(hearting?) the lots in the gallery instead of downloading them all. But there is a problem with that when the lot creator decides to suddenly DELETE his works or remove it from the gallery. I now make it a habit to only download the ones that are truly incredible and are works of art (which I would want to peruse at my leisure).
The rest of them, I only 'favorite'. These favorites can be viewed anytime, just go to your Profile in the Gallery and click on catalog. There is a button there to view 'Favorites'.
I also tried saving Tray folder when I moved from the Philippines to New Zealand. Some things I noticed was that my saved 'residential' became 'unknown' in the info window. And one of them even changed the creator name and didn't recognize me as the one who did it. It took quite a lot of work to transfer every room and rebuild the house.
I would advise that you
1. save/upload your own works first in the gallery itself to retain ownership
2. save your tray to an external drive/usb or save on other locations in your computer
before deleting.
EDIT:
@Playalot I just remembered a trick I do often! I would place the houses on existing plots of land wherever they would fit, save that game, then secure the game file somewhere else with a note describing what is in that game file. This might also work with households. I you want to have them back in your game again, simply load the savedgame and viola! they are there where you left them.
I usually have separate game files for starterhouses, castles, asian houses, you get my drift. Many houses in one saved game.