If we are using ADSL connections, what would the performance be like for backup to the cloud?

To backup to Acronis Datacenter, you can test out the connection speed via this link. The result will give you a rough idea on how well is your connection to Acronis Cloud.

Australia Datacenter - https://cloud-wr-au1.acronis.com/speedtest/ 
Singapore Datacenter - https://cloud-wr-sg1.acronis.com/speedtest/

For the initial backup of larger data over slower link, we would recommend to use the Initial Seeding (IS) approach by transferring the first Full backup via hard drives. IS is available for Singapore Acronis Datacenter currently. As for Australia Acronis Datacenter, the IS service is work in progress.

Alternatively, SP can also consider hosting the Acronis storage for your customers, which will support IS service

For details on how SP can setup the initial seeding and LSR service, please refer to https://kb.acronis.com/content/56070

In terms of how end users can create initial seeding backup at their end, please refer to https://kb.acronis.com/content/57604