Hi Ian
There isn't a simple answer for how your system will perform under load as this depends on a number of interrelated items such as database version, web server used, caching used, bandwidth available, memory available etc.
Totara can be scaled and tuned to perform better - one of our more technical partner will be able to help in this area
There is some documentation on the general approach to use for performance tuning for caching as you can see this is not straight forward.
As far as I am aware the recommendation is the same about cloning a site from database backup.
Regards