I have the similar lag issue with the rest after the March 26th patch. I lag in PVE open fields, WvW, and story quest but don’t lag in dungeon’s and in Lions Arch (lag’s in other towns).
I am playing from Malaysia.
But somehow i found a workaround for the lag. If your playing from Malaysia and using Unifi (that’s what I am using). The lag can be fixed by double logging into the area!
For example, I am lagging in Queensdale, all I have to do is to waypoint to Divinity’s Reach and waypoint back into Queensdale and the lag is fixed!
That works for me anyway just give it a try. Its annoying I know but that’s how i cope with it now hoping Anet would fix it!