Christian Zibreg, iDownloadBlog:

Downloading watchOS software to your Apple Watch is a tremendously slow process.

It can take anywhere between half an hour to an hour or more to send a watchOS software update to your wrist. Because it’s such a sluggish experience, I try to update my watch only when I’m positive I won’t be using it for a few hours, like right before I’m about to hit the bed.

And:

I’ll let you in on a secret: sliding the Bluetooth toggle to the OFF position in Settings → Bluetooth on your iPhone will speed up watchOS software updates dramatically.

But timing matters here. Follow the link for the details. This is an interesting tip. Of course, you can just let the update happen overnight. But personally, I find the details fascinating, worth the read.