This is exactly what I described in the documentation.
Using Scheduled Tasks is slower than any other backup method for the reason I explained (the dead time between task execution). Add to that the fact that Lazy Scheduling only gets triggered when there's traffic to your site —which in your case is very sporadic— and you will very easily get a backup which takes forever to run.
Yes, the Lazy Scheduling SUCKS. Please remember that what Joomla is doing with Scheduled Tasks and the Lazy Scheduling to trigger them is nothing new to us. We had already done that in 2011 with the Akeeba Backup Lay Scheduling plugin (Joomla reused the concept and the name of our old plugin). Having had that experience I knew exactly what would go wrong and documented it.
The only way to have reliable backups —and Scheduled Tasks in general— is to trigger Scheduled Tasks with a CRON job which executes every minute.
Even though it sounds redundant, considering you can already scheduled backups using CRON jobs, there's a good reason to still use Scheduled Tasks: you manage your Scheduled Tasks from the much friendlier user interface in Joomla itself while having only one CRON job, regardless of the number of Scheduled Tasks on your site. If your host only allows accessing a URL (pseudo-CRON), no problem, Joomla Scheduled Tasks can still work with that.
Just don't use Lazy Scheduling. Lazy Scheduling is unreliable.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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