I agree with
@Cofga - hornbeam are understory and forest edge trees, they need shade in the hottest part of the day.
Important - slip-potting is a horrible thing to do to your tree. There is no such thing as a ''gentle slip potting''. No matter how gentle you try to be, 90% of the time the root ball falls apart enough that a lot of fine feeder roots are broken, even if you don't notice the fine feeser roots and root hairs getting broken. This loss of root hairs and broken feeder roots is as traumatic to the tree as a full blown repotting.
Second issue with slip potting. If the mix filled in around the old root ball is different in particle size or composition you will have trouble controlling moisture. Either the root ball will stay too wet, or the outer fresh material will stay too wet and the old root ball may dry out. It can be a nightmare trying to keep the two zones of media in the pot at the right moisture level for good growth. Many a tree has been killed by slip potting.
Third, if you slip pot, the tree will need to recover before you can do a ''regular repot'', this means you are suck with the 2 component mixture in the pot with the difficulties of watering it properly for at least another 12 month. If you repot a tree twice in less than 12 months you will severely drain away its vigor, its resources and risk loosing the tree. Any root disturbance is traumatic to the tree, there is no such thing as a ''gentle repot''.
If you think the problem can be cured by repotting, then do a full repot. Make it count, get rid of the offending old potting mix. Prune roots, shape the nebari, if you are going to traumatize a tree, at least make it count and do proper root work. For this reason it is best to wait for the ideal season for repotting. But there are fair success levels at repotting in the secondary late summer & autumn seasons. So if you feel it is an emergency, then do the repotting, but don't fool around trying to ''slip pot'' that will do more harm than good.
After care is critical when repotting in less than ideal timing. I would wait on the repotting if possible.