Startup is small (roughly 45K), so it's difficult to trim much fat from it. If you use the QNX Instant Device Activation technology, your minidrivers will be linked to the startup program and will consequently add to its load time. So make sure that your minidrivers are as small as possible—don't clutter them up with lots of unused debug or kprintf() calls.
For more information about minidrivers, see the Instant Device Activation guide.