I'm really sorry, because a while back I had the same problem, and GMail was running really slowly, no matter what I did, whether it's opening various emails, performing searches, navigating to other labels, etc. It was this way despite my using Chrome and even Google's DNS. But at some point they either fixed it or I did something that improved things.

I can't remember if it was clearing my cache or what, but somehow I fixed it. If I remember I'll let you know...