Sorry to tell you screencap just a simple command, only accept few arguments, but none of them can save time for you, here is the -h help output.
$ adb shell screencap -h
usage: screencap [-hp] [-d display-id] [FILENAME]
-h: this message
-p: save the file as a png.
-d: specify the display id to capture, default 0.
If FILENAME ends with .png it will be saved as a png.
If FILENAME is not given, the results will be printed to stdout.
Besides the command screencap, there is another command screenshot, I don't know why screenshot was removed from Android 5.0, but it's avaiable below Android 4.4, you can check the source from here. I didn't make my comparison which is faster between these two commands, but you can give your try in your real environment and make the final decision.
