Hi Tom,
Thanks for sharing these performance insights and welcome to the ROOT community!
Lxplus at CERN is a great service, offering a common platform to all users. However, sessions run on virtual machines and those are scheduled to optimise the usage of the underlying hosts: in general, as a user, you are well shielded from others but you rarely are using the computing resources alone. Therefore, any performance considerations made on lxplus machines can not be really trusted.
Cheers,
Danilo