I had one for a week and returned it. IIRC there was no sleep/standby feature. You either lock it like an iphone screen or turn it off. But it starts pretty quickly when you turn it off, so that's not the problem. For me it was just too slow and instable. There is a github repo with all the issues (https://github.com/vext01/hiby-issues) - check that before you buy one.
When I say sleep I mean that you listen until a timer runs out and it pauses whatever you're listening to automatically.
that sounds like a very niche feature, does it exist outside of ultra-tweakable firmware/OS like rockbox?
1. https://store.hiby.com/products/hiby-r3-pro-saber