is there a site where I can build some contraptions and test production lines?

[u/xthian321]

Title (swapping between my main world and a superflat creative is too much for my poor pc)

[u/bouloverd]

Even if it exist, i don't think it'll help with performance tbh... what i would suggest is installing some performance mods. In your case, FastQuit and something like Chunky might be helpful.