Attention: these patches are old and unmaintained. I no longer use DBMail and have no idea if they were integrated and what is going on with DBMail now.Here are my patches that I propose for DbMail 2.0.1. They are independent on one another, as they affect different files.
dbmysql_v2.patch UPDATED. A speedup for mysql db_get_result(). Simplistic; I think it can be considered stable, although there was a bug in the previous version
icfetch_speedup.patch a big speedup for _ic_fetch() ; may need further testing
db_header_speedup.patch another _ic_fetch() speedup; actually concerns db_get_main_header(). Much less significant than the previous one. One-liner and quite straightforward; I think it can be considered stable.
search_uid_bugfix.patch This one-liner fixes the SEARCH UID 1:* issue that made IMAP operation with sylpheed fail.
dbsearch.patch Header search speedup. Needs more testing; relies on a zero-out being interpreted as NULLs.