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Tempest's CPU usage is not real concern even with multiple processes.
The processor mostly consumed by the other processes.

The thing still doable on python level, is event based I/O (select,epoll,libevent, ..) 
 on single thread with multiple connections,
 it can mitigate the number of context switches.

- This is not doable in a regular unit test framework
- Not necessarily all library provides good support for this.
- Tempest normal usage is far from the level, when it really makes sense.
- We lose more CPU power because of other things