Apple - why do I have to double tap command + space to open spotlight?

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.