Three.js can be as performant as WebGL, since it's not hard to drop down to buffers and shaders and sidestep any per-frame overheads.
There are tons of gotchas though, especially for VR render loops. If you naively add even a medium amount of stuff to the scene graph, it's easy for the management overhead to eat up your budget.
On the plus side, it's all Javascript, the code is solid, and it's easy to profile with browser tooling. But Three.js isn't going to do any voodoo magic for you if you don't know what you're doing, whereas Unity might. A double edged sword.
There are tons of gotchas though, especially for VR render loops. If you naively add even a medium amount of stuff to the scene graph, it's easy for the management overhead to eat up your budget.
On the plus side, it's all Javascript, the code is solid, and it's easy to profile with browser tooling. But Three.js isn't going to do any voodoo magic for you if you don't know what you're doing, whereas Unity might. A double edged sword.