Presentation performance notes

[mk]

Benchmarking using the Book of Clerk @tonsky and I found that:

• [ ] image handling is ~25%, should cache it with a type holding the memory image and encoded image
• [ ] prepend-required-cljs is ~10%, can probably record the viewers at presentation time
• [ ] process-blobs is also ~10%, can also be recorded at presentation time
• [ ] bounded-count-limit is very high with 1 million, let's lower it
• [ ] ->hash-str is ~5%, we don't need a valuehash