Cookies? How are you going to store 256+64 tiles+events on 8 layers in a cookie? I would recommend to either save it in a homegrown format and then later on allow downloads by converting it to a j2l file (needs a 1-way .j2l implementation), or saving/loading in .j2l on the server (needs 2-way)

The app feels much more responsive now. Alternatively, maybe it works to not make 256*64 tags but create them only when you scroll. (or rather, replace the contents of those images). I'm a javascript/html n00b, this might actually make the app slower...