I’ve tried your macro, and it launches Alfred’s search box pretty fast, in my view. Only if I start to type really immediately after pressing the hotkey I can reproduce the problem with the first character missing.
I think there’s nothing you can do about it. What you are doing with the macro is rerouting the launch of the Alfred box through KM and AppleScript. This explains the slight delay.
To avoid this delay, and to launch the Alfred box as fast as possible, you should just use the native Alfred shortcut, without any rerouting.
I think the main purpose of Alfred’s “search” AppleScript command is to pre-populate the search box with text, for example tell application "Alfred 3" to search "os x ".
By the way:
I wouldn’t use the Option key alone as hotkey modifier. Unlike as with other operating systems, on the Mac the Option key is not a “meta” key. It serves as character modifier, similar to the Shift key. The key combo ⌥ Space is used to type the No-break Space character (U+00A0) in almost every program that accepts text input.
If you assign this combo as a KM hotkey you are no longer able to use it for typing the character.