There’s a known performance regression with Chrome and Blockly, the library we use for the blocks editor, so at the moment for larger projects Firefox tends to be more usable.
The primary limitation with the number of blocks is the resources your machine has available (CPU, RAM, and graphics processing). The more of these you have, the more you can put into a workspace before it starts to slow down. One trick you can do is collapse the top level blocks because we do have an optimization to not render as much when they are collapsed.
I gave a talk at the Blockly User Summit 2019 that discusses Blockly performance in App Inventor that may be of interest to you: