I feel like no one will be able to answer your question quite yet but I could be wrong. I think the game is being optimized for consoles first so the versions should be pretty similar then. There is a chance some loading zones could still be faster on PC though depending on how they program it. The reason why everyone runs PC is because the toybox / toybox + is downloadable only through Steam. This is just my speculation though so take it with a grain of salt.

Load times on PC are usually faster, especially if you're running the game off of an SSD, so PC will probably end up being the fastest, unless there happens to be some kind of exclusive glitch on the console versions or whatever.

The game is not being optimized for consoles first, as the PC version is the primary one, and the console ports are being done separately by Team 17. Also, the Toybox is available on console, but it runs at 30 FPS IIRC, and has frequent framerate drops from what I've heard.