I think the insight is not incredibly original and follows naturally from OpenAI's Sparse Transformer. The idea is similar to Longformer. Two teams at Google had a similar insight, hence the high number of authors.

The original implementation only took a couple of months and was primarily motivated by internal Google applications. Natural Questions was the first external benchmark we tried to validate on, which took a few months to find the right setup. All the other datasets, took a few weeks but the effort was done in parallel given the large team.

There was quite a bit of frustration dealing with Tensorflow, TPUs, and the XLA compiler that maybe set us back a few months, too.