Is 3MB too big so it makes my computer slow or?
Nope, 3MB is not big at all.
The only way to tell is for it to be available for keen eyes to look at it.
At what point is it slow? As soon as you open it, or after a little bit?
Do you have any plugins installed? Which ones?
Where is the file located?
How many tiddlers do you have open in the story river when you first open the TiddlyWiki?
It could be you have a tiddler open that is really slow rendering and re-rendering itself way too often.
But 3MB should not be a big deal.
Basically due to some plugin that rerender too frequently or runs a complex algorithm during render, like Using lotsof CPU · Issue #10 · bimlas/tw5-locator · GitHub you can use webdev tool to figure out which plugin makes it slow.