Keeping Lag to a Minimum...
I was chatting about a few things I do to combat lag over on discord and realised I do quite a bit of maintenance to reduce lag/load times. I decided to write em all down and before I knew it, I had a big ol' list. Here's hoping it helps!
Save/File Tips:
Clear your caches (located in the same spot as your mod folder fyi) I usually delete the onlinethumbnailcache, avatarcache and localthumbcache caches every time I exit the game, the main one to delete regularly is the localthumbcache file, they're just temporary files but it can get pretty big after a while.. if you're having any mod issues, particularly ui ones, it's always worth deleting that to see if it helps.
Remove any saves you're not using from your saves folder, and keep an eye on save file sizes. In my experience, any saves over around 30-40mb start to get a bit laggy. Things that bump this size up are the amount of townies in game and the amount of lots/objects in the world.
Regarding the above point, I regularly bulldoze lots I don't need anymore, just to save the game from having to cope with extra shit to load in the background and reduce save file bloat.
Similarly, I delete a lot of unnecessary townies. Also, try to keep the amount of outfits on townies to a minimum, ain't no townie need four swimsuit outfits, the game will thank you..
If you have cheats enabled you can usually shift-click/delete object on things like random coffee cups or stray cupcakes sims foolishly drop around the world (or eat em.. whatever tickles ur pickle) just get rid!
Whilst we're keeping objects in game to a minimum.. I try to clear sims inventories now and then, including townies. You can do this via mccc even if you're not currently playing that household by heading to MC Cleaner -> Sell Sim Inventory/Sell Household Inventory. RANDOM LUMPS OF CLAY BEGONE!
Think of a save file like a lot.. the more objects in it, the laggier it gets! Try your best to remove anything or anyone unnecessary where you can.
Hit "Save As" instead of "Save" now and then - the more you overwrite a save, the more chance it has of going wonky, treat your game to a fresh one now and then and remove the old one from your saves folder (maybe don't delete it right away in case you wanna roll back, keep it somewhere safe).
If you can (although ik it can be a pain) set up a new/fresh save, you'd be surprised how bogged down old ones can get! For legacy players, it's usually best to do this whenever you have a small family to save yourself some pain, since you can just save the household and take em to a new save (you will lose sims inventories/household inventories/relationships with sims outside the household tho so beware! Pictures/other collectables can be saved to a lot however, and you can always cheat back relationship bits etc.. bit of a last resort but new saves are shiny and fast!)
Make sure your Screenshots/Video folders are empty - move em somewhere else, it works, trust me. If you have a lot of custom music installed try n' clear some of those out too.. the smaller that Sims 4 directory is, the better.
Delete any last exceptions/last crashes (same spot as your mod folder again) you don't need em unless you're planning to upload em somewhere for help.
Settings Etc:
If you get a bad lag spike, opening the main menu and/or saving can randomly stop this. If it doesn't, try exiting the game, clearing your localthumbcache and restarting.
Clearing all notifications from the game panel can also help.
If you can, close all other apps and background apps you don't need whilst playing, ts4 is super memory hungry so it can definitely help.
The GraphicsRules Override file by Simp4Sims can reduce lag/latency and make your game look a little better in the process!
Srslysims Simulation Lag Fix mod can help reduce lag too (if you've altered the game speed via mccc tho, don't use this unless you plan on resetting it to default).
If you aren't keen on, or barely use a pack, consider disabling it.
Not ideal, especially for those of us taking screenshots, but lowering the graphics settings whilst playing definitely helps.
If you use re/g-shade, consider switching it off whilst playing and only turn it on for screenshots.
A clean and tidy pc/laptop runs better in general. Remove anything you're not using/don't need anymore to free up space, remove temp files, clean up your folders now and again etc etc.. especially that dreaded mod folder, speaking of...
Managing Mods:
SORT OUT YOUR MODS FOLDER FOR THE LOVE OF GOD.. skdsjdjs it doesn't have to be immaculate but at the very least try n' clear it out now n' then. Also try to separate your script mods/overrides from the rest, patch days don't need to be so stressful ;-;
Personally, I don't merge my mods; if something breaks it's much harder to pinpoint! It makes it easier to find/delete specific mods too.. and let's face it, there's usually one or two items in that set you could do without lmaooo.. also, don't think it helps much tbh! Yeah you could say the game doesn't have to work as hard to load merged files but that's debatable, it's still the same amount of items/polys at the end of the day ¯\(°_o)/¯
If you like merging files and/or see results from doing so, you can merge stuff you definitely know you're never getting rid of, especially CAS/BB stuff.. but steer clear of merging gameplay/script mods! If a merged file seems to be the culprit when using the 50/50 method, try unmerging it and 50/50ing it again! You might not need to get rid of everything if something's borked.
Bulk Rename Utility can be used to remove all spaces and special characters from your mod files, the game doesn't particularly like loading those so it'll thank you.
The Sims 4 Mod Manager is a great way to sort through your mods, you can easily see, move and delete files from here (not great for build/buy, poses etc as thumbnails are usually missing, but great for CAS stuff). An extra hint with this that I've noticed is that if any of my mods get renamed with [D1] at the beginning after looking through them via the mod manager, it means it's a duplicate file so you can get rid.
I also use the Sims 4 Tray Importer to help me sort through mods. Simply save a sim/lot with any cc you don't want and find it in the importer, you can then go through all the cc in the cc tab and delete/sort it (I also use this to sort cc if a bunch has the wrong tags etc, makes it easier to find in my folders by saving em to a lot or w/e - it also spots duplicates which is handy).
It's a ballache, but the 50/50 method is tried and tested if you're having issues.
I like to keep an abandoned cc folder tucked away somewhere, that way you can remove mods willy nilly without stressing about losing them. If you change your mind, you can always grab it back!