It's also a very good example of what and how to escape shell special characters in apache log pipes :) (I spent a fun morning with a test apache installation and a perl script testing each special character in turn until I got it working...
Which could explain the performance issues you're having, especially near the end of the import process.
This dispatching was required to make sure requests are imported in the correct order.
The drop-off seems to start roughly half way through any given logfile.
oliverhumpage: 48 is almost certainly too high, unless you have a 48-core machines. On my system, I have a sustained 300 req/s for more than 3 hours.
oliverhumpage: thanks for the bug report and the suggestions, I've normally committed everything you asked :) Regarding the persistent connections, I haven't patched anything.