If performed inefficiently, excluding files from a cloud backup can cause backups to take longer.
Cloud backups rely on R1soft, a block-based backup technology.
This technology works well with a low number of exclusions but is inefficient when a large number of files/folders are excluded.
If your backup takes longer than four hours to complete, iWeb recommends reviewing your backup policy: https://kb.iweb.com/hc/en-us/articles/230263988-Creating-a-Backup-Policy.
If you have folders with over 100 files that you wish to exclude from your backup policy, you may want to consider creating a partition just for this folder. The partition will be excluded entirely and will not take any time to be excluded.
However, please note that if you do not have any available unpartitioned space on your server, you will have to resize existing partitions in order to create this new partition folder.
Resizing partitions is risky and should only be performed by an experienced systems administrator.