While knowing the time when the site's traffic is the busiest can help you with exposure, that isn't a guarantee it will help your story catch a Trending spot or increase the readers.
ScribbleHub is fairly well-known in the writing community (often paired up with RoyalRoad), thus, many of its users and audiences not only come from the Western half of the globe, but including the East as well. Hence, a definitive 'busiest site traffic time' is up for speculation and observation.
I know some authors whose work gained followers at different times of the day, as well as a stretch of several months, sometimes even years. There are also those who slowly got famous for uploading consistently, and getting known through 'word-of-mouth'/recommendations.
This is why I'd like to raise the point that the answer to this thread will depend on the day and time your potential readers will come here. That's why, it's safe to recommend to upload your chapter at different times and days, so you can maximize its exposure, instead of just a set of hours.
I mean, we can set the automatic release of chapters anyway, why not use it?