...
- Enable instant auto-cleanup of uploads and downloads.
client-> click on a folder->Transfers->auto-cleanup interval ->Immediately - Reduce the number of files in your folders.
- Split up big folders into smaller ones.
- Don't sync/setup folders that do not actually change.
- Cleanup the folder database.
client-> click on a folder-> Settings-> Cleanup Database
To perform the tasks above it is required to set up the client to Expert Mode.