Why are high res screenshots make a terriable performance when playing game

i want to capture the screen when playing game。but the performance is terriable。
I have uesd postprocessvolume in my game.And i open all the ray-tracing effects.
i set the “Ray Tracing Global IIIumination” values{
“Tyte”:“Brute Force”
“Max.Bounces”:“1”
“Samples Per Pixel”:“4”
}so that it look better in editor.
But when i Play the game,many noise came out !
That is why i capture the terriable screenshot!
there are two pic between playing and stoping.


playing

307415-微信截图-20200717175832.png


stoping

307416-微信截图-20200717175810.png

I want to know why it is different。
And how can i get the second performance when playing game?