The fastest way to break glass would be to -

Hold out your Medkit then look towards the glass, hold Shift the entire time while running towards the glass, jump then throw it exactly when you jump. (Keep holding shift even after you've jumped to increase speed)
(Press T to throw)

Using this method will only take 3-4 tries until the glass has broken

https://youtu.be/mwPcVw_dHZE


Keep in mind that EVERY item in the game can break a window, so if you only have a keycard then you can still try this method it will take a little bit longer though

Light objects: Keycards, SCP 500, Pills , Adreniline etc will deal low damage (This will take a few tries to break the glass)
Medium Objects: Medkits will deal medium damage (This will take atleast 4 tries as shown above)
Heavy Objects: Weapons, Torch, etc (This will take 1-2 tries to break the glass)
(If you find a Torch and or Radio in Light Containment, it'll take 1-3 throws before it breaks the glass)

This guide is mostly dedicated towards the very early game when you're either D-Class or Scientist and want to break it to get the SCP item inside or just break it so you can get it later once you got a high enough keycard.

Remember this works for ANY window that has breakable glass and speed is the key the faster you go, the faster you can break any glass.

Good Luck!