If the scene first have to be transferred and then loaded, it will of course take time before the slave can start.
I have an orbx file 429MB in size. When I load it in my Master and hit the render button it will take 28 seconds before the slave starts to render. If I then turn the camera the slave will pick it up immediately and start to render. So it is the loading time that is the bottleneck. When the scene is already loaded on the slaves, the render should start immediately on the slaves.
I don't think you will ever get a 2 second render with loading time included when using a slave. Unless your scene is small like 1 polygon
Instead build a PC with as many really powerful gpus as you can. Then you can skip the bottleneck by having the load time on the slave.
So if you need a render in a few seconds use 1 really powerful PC.
If you can wait a few minutes then a slave can be used. But be aware of the loading time.