Effect of travel adapter on powerline

I've recently moved from the UK to Canada and am currently using UK powerline adapters plugged into UK to US travel adapters. The speed I am getting is around 3-5mbps, compared to 15mbps at the modem.

My question: am I likely to see any improvement in speed from purchasing NA powerlines (i.e. are the travel adapters the issue?) or is this just likely to be crappy house wiring?

I know that powerlines do not like being plugged into extension leads, but thought the adapters may be minimal enough to not have too much of an effect. Has anybody got any experience with this?