I have been having the same issue as everyone else. Today I was using the x2 with no charger all day and the tablet stayed at 100% while the keyboard drained throughout the day (it dropped 14% while I was at lunch!). Once the battery was between 4 and 2% I noticed that the tablet was down to 93%. I pulled up my battery settings (there are 2) and noted the low/critical values are 4% and 2%, Respectively. I adjusted the battery settings to something like 80/40% respectively, Left work, came home doing more work and the keyboard is still at 2% while the tablet is now at 68% over the course of 3 hours.
It might not help slow the drain once fully charged but I plan to test this more. t might be something you want to try.