I had complained earlier about mongoHQ crumbling under 10 requests per second, but I discovered it was my fault (as usual).
I neglected to set auto_reconnect for mongo-connect (which is just used for sessions — I wasn't sure how to give it a reference to mongojs's connection, so it opens it's own connection)
When I tried again, setting this flag, with only 2 dynos, the server was handling 450 requests per second before crumbling.
With 4 dynos, it still seems like it dies at about 400-500 requests per second..
With 8 dynos, the same.. so I'm guessing the database it happy to handle about 400 requests per second. Anyway, that's a lot more than 10.
I need to update this setting in february-fire and nar-nar..
nar-nar done, and it still works.. good sign..