The default implementation of Reader::readBytes on 1.3.2 reads bytes into the result vector one-byte-at-a-time. For large byte arrays (~500k or so), this is horrendously slow.
The code can easily be changed to simply do:
..which will copy all the bytes in a single call.
(note: it appears this function has been changed in the trunk, but it still copies byte-by-byte, so the optimization would still apply).
In my testing of serializing/deserializing a message with a 500k byte field in it 1000 times, execution time dropped from from 30+sec to 0.2sec with this optimization.
The same optimization can easily be applied to readFixed(uint8_t *val...) as well.