As Gilles commented, you can turn up kernel and udev logging. Arch Wiki has a great source for this. Consider using these kernel parameters at the very least:
debug ignore_loglevel log_buf_len=10M LOGLEVEL=8 earlyprintk=vga,keep udev.log_priority=8
Sounds like your OS is on an embedded system, so YMMV with a 10M log buffer. Adjust accordingly.
Last year a Gentoo user scaled his boot sequence down to under 2 seconds by digging through his boot process for arbitrary
sleep calls (among other things) and replacing them with relevant synchronization primitives. If this inspires you, then as you profile, discern the code path used to mount your usb drives, and review its source code for arbitrary
It's difficult to provide a definite course of action without more details.
- What OS are you using? What version?
- What version of udev?
- Which mount program are you using? What version?
If these options are too low-level, you might try replacing parts of your distro with alternatives. Look into mdev as a replacement for udev (assuming you're not using systemd), or edit your automount udev rules and swap out the mount program for another utility (mount, pmount, udisks, udevil, etc).
Other than that, Google, read, probe, tweak, and measure.
Find below some possible reading material about automounting with udev.