It would require to build a separate way to load the rules from the hard drive.
Currently it is done asynchronously, meaning Firefox continues its loading process while the rules are being read from the hard drive.
With the new preference turned on, Firefox should wait for the rules to be ready before continuing. There is a big difference in programming between synchronous and asynchronous I/O*, and if we are to read the hard drive synchronously it must be implemented separately, as a different branch in the code.
Depending on the needs this branch can be small or big, but whenever you have several branches, you must update and test all of them any time you add a feature affecting that part of the app. So it's not an ideal solution. I hope they won't give up on that please_kill_startup_performance pref though
* Input/Output. For a hard drive it's the read/write operations.