@jihye.jang - The technical maximum number of user connections on a single board at one time is 1,000 as stated in the How Many Users Can Collaborate on a Board Simultaneously help center article.

While 300 participants is only a little over a quarter of that maximum, I suggest that you have a read over the Tips to Improve Board Performance article as well.

My personal suggestions:

- Turn off collaborators' cursors

- Use the Chrome browser (Firefox has always seemed more sluggish for me with Miro) and limit the browsers tabs to Miro only if possible

- pre-install a Miro desktop app as a backup should browser performance become an issue