The --delay-updates option results in 6G memory usage per archive mirror for a total of ~18G memory used on gemini when all three archive mirrors are syncing. Less important (but still revelant!) is the memory usage on each mirror, which climbs to about 11G during each synchronization.
Removing the --delay-updates option should be OK considering the archive hosts data that almost never changes. Without this option, rsync is able to do a sequential scan which uses 90M of memory (per archive mirror) on gemini and about 250M on each mirror individually.