Gave a shot to the new Bundler 1.4.0RC1 during the weekend and found out it now supports gem installation in parallel. To invoke you need to pass --jobs parameter with number of threads you want it to run – for me the best performance was achieved by specifying the number physical CPU cores.
I've tested on the AppFab app. In my case (CPU with 2-core i7 with HT) the speedup was 50%. I've also tried with a number greater than the number of physical cores but the performance was 15%-20% worse.
Bundler 1.3...
TL;DR: If using :group => :some_field you might want to :order => 'NULL'.
According to the man
By default, MySQL sorts all GROUP BY col1, col2, ... queries as if you specified ORDER BY col1, col2, ... in the query as well. If you include an ORDER BY clause explicitly that contains the same column list, MySQL optimizes it away without any speed penalty, although the sorting still occurs. If a query includes GROUP BY but you want to avoid the overhead of sorting ...