With version 1.7.0, the only one limitation on remote sites is the main gallerie capcity to manage the big number of images.
Today we already have synchronized 8 000 images.
If main gallery can not synchronize a remote site with an max_execution_time error, then synchronization should be cut in many steps :
Synchronize only directories
Synchronize directories and files, reducing to single existing categories (repeat this step for each categories)
Synchronize files metadata, reducing to single existing categories (repeat this step for each categories)