Presentation performance notes

@tonsky

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

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