When playing HC, I normally sat at a stable 210ms with occasional spikes over 300.
The last month, playing on SC servers, I've been at 250 at best, normally closer to 300 and regular spikes over 500ms. This has been constant for the last 2 weeks.
Tonight I finished my 5 x 60 SC, suffering from 350-400 lag all evening and switched to my HC char and instantly am back to 210ms stable speed. Switch back to SC and it's back to 250+
Clearly server load does play a part in the latency issue.