I’m sure that was frustrating. (By the way, I’ve moved this to the projects area of the forum so you can get individualized responses based on your own set up and requirements.)
First things first, motion sensor placement. Take a look at the following FAQ (this is a clickable link)
See if that gives you any ideas to start with.
Second, brand of sensor. There have been three or four models of the SmartThings brand motion sensor, and the most recent has turned out to be the slowest, unfortunately.
The Lowe’s iris motion sensor is quite popular and seems to be one of the fastest of the inexpensive ones.
Another option to consider is the Phillips hue motion sensor which is very fast. You can set it up one of two ways, either just connected to the hue bridge without bringing smart things into the picture, which will probably be the fastest, or using a custom device handler from the community which let you connected to the smartthings hub.
But no matter which one you pick, all of the advice on motion sensor placement will apply, so do read that FAQ first.
At my house, we use the Hue motion sensors, with the hue bridge, not with the custom code, and we get the fastest response that way, just about 400 ms (just under half a second). But for various reasons different things may work in different households.
(By the way, I’ve moved this to the projects area of the forum so you can get individualized responses based on your own set up and requirements.)