The mysql-test-run suite is much slower in 5.5 compared to 5.[123] in a
debug build. Like on the order of 5 times slower on my machine (with --mem
--parallel=8).

I remember profiling it some time back, and finding most of the time being
spent in a single InnoDB function. I believe it is some debug check done
inside innodb that consumes huge amounts of cpu time during server start.

I would like to discover the reason for this and fix it so that we can get the
test suite to run significantly faster.