Do large trays cause lag?

There have definitely been times in the past where user tests by Simmers have found that large Tray folders and stuffed Saves folders cause lag. You can drag your Tray folder elsewhere, as well as saves you aren't playing, and put them back when you need to access those things. You don't need the Tray folder during gameplay; anything you saved there or got from there will stay in your save if you move the Tray files out. The Tray is *only* the Library, which is outside your save.

Sometimes deleting the file localthumbcache seems to help, especially if it's been a really, really long time since you did that.

Some lag (mostly during lot loading) is caused by having lots of Sims in the world with lots of relationships -- it's a risk from turning relationship culling off via a mod. Lots full of interactable objects can also cause lag.