25 minutes?! I thought my event sheet took a long time to open at 10 minutes. :(
There's no other way to edit events in CC. This issue that you've encountered is one of the reasons I strongly recommend people use C2 instead of CC.
The only idea I have is to try splitting the event sheet into multiple sheets and use includes. That improved the opening and editing speed on my game, but it significantly increased the compiling time. I don't know if it would do the same for your game, but it's worth a try. Save a copy first!
Also, if you try that and it isn't better, then go back to your previously saved copy rather than copying and pasting everything back to the way it was and continuing! I tried that and it resulted in the .cap being slower to work with even though it seemed like it should have been the same.
Also, try to keep from adding more objects when possible... I've found CC doesn't like having so many of them.
You could also upgrade your computer, as a faster processor will definitely help (I've tried editing my game on a faster computer and it's vastly improved) but that's an expensive option.