A alternative to this has recently been published at ICML that claims to be faster. The website and tutorial video are very nice, too.
I wonder how much of a limitation it poses that the gradient requires some massaging to preserve efficient training; does that only work for some kernels, or can it be automated for arbitrary kernels?
[edit] Linformer (https://arxiv.org/pdf/2006.04768.pdf) is a different project from the one linked in https://linear-transformers.com/ (Transformers are RNNs https://arxiv.org/pdf/2006.16236.pdf).