The first question to ask is why is it slow? Keyboard Maestro runs scripts simply by using the shell command osascript to execute them, so you could try in the Terminal running the command:
osascript /path/to/script.scpt
And see if that is fast or slow.
Also, since Macro Groups control when macros are active, rather that the If Then Else you have, you should have a macro group that is targeted at Outlook and place the macro in there, and then just have the single Execute AppleScript action in the macro. Not that that will make the macro run any faster.
Failing all that, you should try to get an Activity Monitor sample of Keyboard Maestro Engine to see what it is doing, since there is clearly no reason it should be taking six seconds to run a trivial script.