These increased compilation times, when the Burst package is installed, are actually expected (but obviously not ideal).
Specifically, the increased compilation times come from the extra IL postprocessing that happens when the Burst package is installed. And the reason why {{--burst-disable-compilation}} doesn't make any difference is that this IL postprocessing always happens, even when actual Burst _compilation_ is disabled. Because of the way incremental builds and caching works, it's always necessary to run Burst's IL postprocessing, even when Burst compilation is disabled.
The repro steps - e.g. triggering recompilation of an assembly that is referenced by 99 other assemblies - represent a sort-of worst-case for this, because all 100 of these assemblies need to be IL postprocessed.