it just means, they didn't know what was going to happen and in that moment is in a relaxed/not ready position/stance
Making their response too late as they weren't ready for it to begin with when it started happening
Their reaction speed doesn't get slower but their actions of what to do for it does
think like this... You are imaging me going to turn of the light from the switch that's behind you, since you don't know what I was going to do, there's nothing to respond to until it's too late to were I poke your eye and you start realizing what I did/was doing
Hopefully you understand what I mean
So there isn't a way of measuring reaction speed while off guarded unless there's like some statement of how fast their reaction speed is when sharpening their senses (fully on guard) and then saying he got me using half of my strength/senses or something... Basically on very few situations will you be able to measure that