Faster than anything in the Wainer hierarchy (which ends at $f_{\epsilon_{0}}(n)$. But it's reasonably easy to define a fast-growing hierarchy that uses the Veblen hierarchy of ordinals for the subscripts. Then TREE is $f_{svo}(n)$ where $svo$ is the limit of the finitary veblen sequence as the number of elements goes to $\omega$ (aka the small veblen ordinal).