The issue here is probably not the question of SmartThings running locally or not locally, but the “man in the middle” integration with the raspberry pi.
The first thing I would do is ask in the Lutron integration thread and see if other people are either having the same issue or have any suggestions for reducing the latency.
It was announced in January at CES 2017 that there is an official Lutron/smartthings integration planned for release in the next few months, and that in itself might speed things up. It’s still a cloud to cloud integration and you would still need the Lutron SmartBridge, but it may be better tuned.