Have you tried tuning settings below (Is it useless) ?
- "Admin" > "Spam" > "Rate limit for uploading files (per IP/hour)" (Default is 20)
- "Admin" > "Spam" > "Rate limit for uploading files (per user/hour)" (Default is 10)
There is a problem that these settings restricts irrespective of the user level. I think this should be improved ...