VM have publicly stated that they don't and won't throttle, so far every time anyone has suggested that VM has throttled xyz, it's turned out not to be the case, it could be a large number of other things, usually its a local user issue, other times its a remote provider issue, occasionally its a routing/peering/congestion issue, but unless VM are just picking on you, if they rate limited SSL traffic/ports lots of people would report similar issues. Unless your queue is approaching 10K+ items, don't worry about headers, generally you aim for the minimum number of connections to saturate your connection, most of my providers run 8-12 connections on a symmetrical gigabit pipe.
