Big delay looking up blogs with wpmu versus buddypress

I've got two blog servers running. I've been optimizing them, and just noticed a big performance difference between the Buddypress installation and the regular blog server not running buddypress - amounting to an average of 5 seconds delay, apparently for the main blog to look up the sub-blog.
The difference is striking - And I'd sure like to fix it.
Here is the time analysis for the regular blog - note the 6 seconds to access the original blog versus the speedtest-1 (hosted under buddypress):