I can solo clear high 30s grift in ~4 minutes, but I can't speed clear it like I can a T6 rift. Still have to pay attention and play cautiously, so I still T6 for farming items. I'll probably go back to 38 grifts when I get bored and need to switch something up.
And they REALLY need to remove prerolled sockets from legendary weapons. Basically never going to get an ancient BK sword with the affixes I want(IAS, LoH, socket) because of that stupid oversight. Let alone getting two. This stupid restriction is what keeps me from speed farming grifts instead of T6.