By default when you bring in a Sub-Assembly into your assembly it comes in as "Rigid", fixing it's shape so that the main assembly does not have to calculate the mates of the sub-assembly, reducing the require processing power. You can make the sub-assembly move by making it "Flexible", the option is found by right clicking on the sub assembly:
Mates can restrict more than one degree of freedom, therefore the more mates it has can exponentially increase the processing power not double it.
Hopefully this helped you,
i stumbled across feature this a few weeks ago,
was a great help for altering a few things higher up the assembly tree.