Yes. You can grab the ancestors shared and the leaf or subtree you want to modify exclusive. If all of your writers land in the root of the tree because it's a small tree, they all contend on the same lock.
I'm no expert, but it seems like you need to guess at some key ranges and start the BTree at some minimum size even if most of the leaves start out empty. Either that, or don't write anything until the tree grows a bit and rely on a writeahead log for durability.
Hotspots could still show up further down, though, if writes aren't fairly random.
The issue seems yo be that writes in Fractal trees always go through the root node. This would lead to a situation similar to small B-Trees where writes will most likely access the root node and contend.
