@Cliff-Karlsson
I'm not too sure you will get that reaction time from anything but the Wemo app itself, or the mechanical switch on the Wemo. The problem I found out with the Wemo is, it will randomize what port it connects to in a certain range. I can't remember the exact range (maybe 49152-49156?), but I think it's stated in the .sh script. Therefore, the script or whatever program that is doing the switching needs to first find the proper IP/Port, then actually run the command.
The one option you may have is setting a static IP to each Wemo switch, then sending it more direct.
To be all honest, this is why I switched from Wemo over to MySensors and the Relay sketch. Works much better than the Wemo's do, and haven't had a chance to buy and test any Zigbee wall switches yet.