Just looking at the 2 B_2 queries, I'm curious as to why is the execution plan different between the 2 machines. Is the optimiser stats updated on both databases?
B type set are transactions, so it's impossible for me to post EXPLAIN ANALYZE results. I've extracted two querys from a single transactions and executed the twos on both system. Here are the results:
Looking at the 2 B_2 queries (since they are so drastically different), the in-memory quicksorts stand out on the Dell as being *drastically* slower than the disk-based sorts on your mac-mini....