Though I did not find it that slow as you described it, it is true that a shortcut would be faster. Since there is no standard shortcut set, you set it for your self.
There is a search bar where you can search for split view. Then you see the possible commands where you can attach a shortcut to it. It will warn you if you want to set a shortcut which is already in use.
After you set the shortcut, where I would recommend including the shift key, you can mark the two tabs and press your shortcut — voilà! You have your split-view shortcut set up successfully.
I recommend including the shift key because, you need to select anyway two tabs at the same time with the shift key. When you have done this, you just need to press, probably, one key more to have split-view activated. As already mentioned, it’s not a must, just a recommendation.