Probably not worth the effort to try and improve on any current GROOVY custom device handlers or GROOVY Smartapps. Groovy is going to cease at some point in the near future. You may want to keep an eye of the current Beta release for Edge.
You should definitely start looking to the end of webcore and what impact that will have for your pistons. When groovy ends so will webcore. So plan your path now and explore Automations/scenes or Rules API as a means to replace your webcore pistons. SharpTools is also an option with their Rule Engine. Also check out Samsung Automation Studio
I just finished moving all the functions in a webcore piston to handle modes and presence over to automations this week. While I enjoyed the ease of one piston, it required multiple automations to accomplish the same functions.
As for the app slowness, I believe things will improve when the transition from groovy is finished. That is my hope.