Wondering if you got this working? I'm trying to also create a custom glyph in the web snapshot balloon annotation (with the glyph being a base64 encoded svg). I was able to do this in balloon annotations in the mapkit.js map, and just trying to match the styling in a web snapshot.

I'm also seeing similar performance issues in the latest versions. I've tried to adapt by not displaying annotations until I've zoomed into the map close enough that the number of annotations displayed will be < ~300. This is still a major barrier in trying to use mapkit.js in a web application and getting the performance closer or even just displaying groups of annotations when there's a large number that cannot be displayed effectively would be helpful.