Player can move veryfast when Elytra opens instantly.
https://youtu.be/75obI4gzgA8

code in EntityPlayerSP:
//if (this.movementInput.jump && !flag && !this.onGround && this.motionY < 0.0D && !this.func_184613_cA() && !this.capabilities.isFlying) {
if (this.movementInput.jump && !this.func_184613_cA()) {

ItemStack itemstack = this.getItemStackFromSlot(EntityEquipmentSlot.CHEST);

if (itemstack != null && itemstack.getItem() == Items.elytra && ItemElytra.isBroken(itemstack))

{ this.sendQueue.addToSendQueue(new CPacketEntityAction(this, CPacketEntityAction.Action.START_FALL_FLYING)); }

}