Screen Capture from Appium are too slow

Our testing taking screen capture every second to do some analysis but found that it takes about 5 seconds to get 1 image. Our architecture is that the testing python program (which do only screen capture every second) and appium server run in the same laptop (surface pro 4 if it matters) We tried to (1) reduce the reosolution (2) using USB3.0 cable + type c cable + usb3.0 (in surface pro) with Samsung Note 8, (3) reduce log level, but still cannot improve the speed. Can any tell me what the bottleneck actually is and how we improve the speed (to within a second).

testing spec: appium version is 1.6.3 surface pro 4 samsung note 8 usb 3.0 cable

It is a major bottleneck for us now. we are really appreciated for your help in advance.