I think they are complementary, since one is about browser site isolation, and one is about process isolation on the computer.
Using temporary containers, multi-account containers, site isolation, along with a number of other privacy/security addons such as Umatrix, LocalCDN, and many others, I have not noticed any slowdown.
So far it seems to work fine for me too. Can you share you list of security/privacy addons? I've used Umatrix but never heard of LocalCDN. Was wondering what other gems you may have found.
Here is a complete list of security/privacy addons I am using, a small number of which I have disabled, as I regularly toggle them depending on what I am doing at the time
Enable Strict Enhanced Tracking Protection setting, this turns on Dynamic First Party Isolation which is the native version of what Temporary Containers is aiming to do.
Using temporary containers, multi-account containers, site isolation, along with a number of other privacy/security addons such as Umatrix, LocalCDN, and many others, I have not noticed any slowdown.
This on an older broadwell i7 with 32GB of ram.