At the 1/2 mph speed (.13734 m/s) you used with 1 mm deflection, the G force would be 0.96 G's. That is like gently lying down on a hard surface. Your head at rest would be 1 G.

The deflection is the amount of distance the glass door or wall moved combined with the flexing of your skull. If the total deflection was .5 mms, then the G force would be 1.96 G's. If the deflection was .25 mms, the G force would be 3.85 G's.

If you could measure it, you would find that a glass pane has quite a bit of flex. Just watch the reflection off a glass pane during a wind storm. The pane will bow in and out. Plus, there would be some movement in the glass frame in the door track. Even a wall has flex.