The other problem with container tabs, which isn't really a Mozilla problem as much as a Google problem, is that if you have enough segregated google profiles going you seem to hit some google anti-abuse codepaths or aomething. I have 3 separate Google accounts separated with container tabs (on purpose, I know Google supports multiple accounts), and a separate regular browsing container, and occassionally just following a link from Gmail or hangouts where Google bounces you through their redirect hangs for 30 seconds or so. When it's doing that it generally does it consistently though.
The gmail/gdocs slowdowns are being tracked and worked on. It's a problem when websites actively optimize for competitive products though.