When booting ReactOS up to desktop, the kernel makes no attempt to lazy flush the registry hive down to disk. This is because the CmpLazyFlush lazy flusher function gets called frequently when unlocking the registry with CmpUnlockRegistry. This phenomena happens when parsing a key, for instance.
As a matter of fact, repeatedly calling KeSetTimer only keeps removing and inserting the KTIMER object and never gets the chance to elapse the time and fire up the lazy flusher worker engine. On USETUP phase installation the lazy flusher gets the chance to fire up and actually do a lazy flush because during that phase installation the setup doesn't perform registry operations that much, unlike being on desktop.