In the beginning of the session, i load up a very big amount of transform arrays in my gamemode, using load game from slot nodes. The process takes up to a minute to load, so each time I test the game, I have to wait up for a minute. Is there a way to optimize this?
Now delays won’t work, because cause bottlenecks in performance throughout gameplay
You can use the async load, but you can only perform ONE operation with that on each save game, and you need to wait for it to finish before initiating another.
At least you won’t have to wait 
yea sync LoadGameFromSlot on huge arrays will do that, it blocks the game thread. switch to Async Load Game From Slot so the load screen stays alive while it streams in (one op per save object at a time, wait for complete before the next). also worth splitting the data: player-critical stuff loads first, bulk transforms stream after, so testing iterations arent a minute of staring.
Advanced Save System, a save/load system plugin for Unreal Engine 5, has async save/load built in with encryption and cloud saves: Advanced Save System. on sale ($23.93) if you want that afternoon fix.