There is a lot on this topic already in the forum.
In a nutshell: normalize and cleanup your animations in Blender. Also have a look @labris optimizer! This will drastically reduce animation file size.
Blender uses a different timeline than Babylon. A marker on keyframe 3 in Blender, does not readily translate to Babylon getCurrentFrame(). I cannot quite remember whether GLTF supports animation events.
In terms of the amount of data. There ususally is 1 rotation per Mixamo bone (~70) per animation frame. That will be 2100 rotations for a 30 frames animation.