First, Switch is deprecated and should not be used, although your numbers imply this is not the issue.

Find out what your USER_DBI modules are doing at compile time: the number of lines of code is a poor indication. They might, for example, connect to a database in a BEGIN block and wait for a DB service to respond. They might be connecting to another machine through a socket; they might be calculating the number of molecules in the universe in a BEGIN block for all I know. There are several other blocks that get executed at module load time as well, and they could be using any or all of them.