why do I have to double tap command + space to open spotlight? Since I installed High Sierra I sometimes have to double tap Command + Space to open spotlight search bar. This problem is weird because it happens randomly. Say I just double tapped Command + Space, it won't happen again for a couple minutes.  

I haven't setup any shortcut nor I use any app that steals this shortcut. I have no idea what is going on and it is really boring, cause sometimes I'm in a text editor, tap Command + Space, nothing happens but as I'm used to it, I start type right away and mess up my code.
I had the same problem. Apparently, Siri is waiting to see if it's being called. In the Siri preferences, there's a setting for Keyboard Shortcut which is set to Hold Command Space by default. So sometimes you might be pressing a little longer but not long enough for Siri to kick in.

Change this setting to Press Fn (Function) Space as shown in the screenshot.