Yes, especially with fast hashes like MD5, a slow Internet connection definitely makes a difference. I speak from Alaskan experience. 
You might switch to only using --brain-client-features=2, so that individual hashes are not synced, but entire attacks (such as a specific mask) are.
You might switch to only using --brain-client-features=2, so that individual hashes are not synced, but entire attacks (such as a specific mask) are.
~