How to Have More Than 8 Sims

You cannot move sims between neighborhoods, because in Sims3 (unlike in the Sims 2) the Neighborhood is the save file*.

What you can do, is upload the family in question to the House bins, move to the new neighborhood, and then place them from the housing bin back in the world. However, this eliminates all non-household relations for the sims in question.

*Though I suppose the whole point of the World Adventures expansion is moving to foreign neighborhoods.

The Sims 3 is notorious for high CPU and resource usage, especially when you get multiple households and things going. Try opening up the task manage (ctrl+shift+esc) while playing and take a look at the CPU and Memory percentages. If they are pegged at 100% or close to it, that may be the problem.

The game does try to "optimize" by instead of the entire game chugging, only certain processes of it do, like sim behavior and the background world simulation (which is why everyone is tired). Something to try is lower the various settings, or removing households you don't really care about so the background simulation doesn't have to work as hard(backup your save in case this doesn't work!).