The issue isn’t the device. It’s the code, both the device type handler and the smart app.
SmartThings is primarily a cloud-based platform. With version two of the hub, they did allow for a very small amount of local processing, but it’s a very small amount. Only device type handlers which are included in the firmware for every smartthings customer are eligible to run locally. And the only smart app which can run locally is the official smart lighting feature and a small amount of smart home monitor.
And that’s it. Routines don’t run locally. The mobile app on your phone cannot talk to the hub locally even if they are both on the same LAN. Any custom code, whether it’s a device type handler or a smartapp. Will not run locally.
There’s no official list of DTHs that run locally, but there’s a community FAQ:
You can see that the SmartThings outlet is there–but only if you use the stock DTH and only if you are using a smart lighting automation which only includes devices using DTHs on that list.
This isn’t what we were expecting when they said V2 would have local processing, but it’s what we got. 
https://support.smartthings.com/hc/en-us/articles/209979766-Local-processing
In your case, if you use the stock device type handlers and you use the official smart lighting feature to have the pocket socket turn on and off when the switch turns on and off, it would still work locally.