Hey, thanks for playing and the helpful feedback!
The Great Creator is indeed the final boss. I believe the idea was originally for the player to walk up to the computer after defeating him and have the credits roll, but unfortunately some last-minute changes and git problems led to us losing the credits scene and re-using the clock elevator in the starting room as the exit trigger (I'm assuming it didn't break for you somehow). It just takes you straight to the main menu though, so you didn't miss anything there.
The controls are shown in the settings screen and on the Itch page, but showing them in-game like in the starting room before the first boss level would probably be a good idea.
We made some late balance changes to try to prevent the bosses being too hard, but that ended up backfiring a bit because we didn't have time to playtest them properly. Definitely something we want to address in a post-jam release.
And yeah we need to look into optimizing/reworking the Marionette attacks haha. We used an asset to draw the pixellated string attacks and didn't have time to change it after realising the performance issues. Enabling hardware acceleration in your browser might help though, if you hadn't already.