What kind of tenseness?
If the horse is getting frazzled, like jumping around, not staying straight or on the circle, etc I think it's because they need more prep work. I tell this to barrel horse people a lot. Build speed gradually, don't go from a slow lope and then blastoff, or you'll get a little too much horse on your hands. Go to the speed just where they start to feel a little uncomfortable, hold them there briefly, and then ask them to slow down again and ask them to round up, get soft, bend, counter canter, anything really. With Selena we like to push, push, push, then immediately drop down, change leads to the counter canter, reverse the arena at the counter canter until we are on the correct lead again and break to a strong working trot with bending and flexing, etc. I just find it keeps her thinking about doing something right after she runs around. The horse has to stay with you no matter what in those situations.
If it's just a matter of the horse not wanting to slow down, I will do the same kind of prep work by building speed slowly and making sure I have a rate command. That's pretty interchangeable. Selena knows "easy" as her slow down command. Even at the walk I will say "easy" and expect her to stop. If we're trotting and I say "easy" I expect her to walk or slow the trot to a jog. Loping and "easy" mean trot/jog, and when we go on large fast circles like that "easy" means come back in and circle slow. This is accompanied by a cue from your seat. You can't be afraid to get after them either if they don't respect that cue. Ask as soft as possible and then they have exactly one second to respond, before I pull on the reins and make them back up, away from my hand.
Hoping that helped. Selena's large fast circles and speed transitions actually came more from her gaming, when she learned that running was no big deal.
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.