Is my upward sloping flush pipe giving my loo a weak flush?

The flush pipe from the bottom of my concealed cistern starts too low so enters the wall mounted loo at 5º below parallel to the ground. Is that enough of an incline to give me a slow weak flush - water fills the bowl slowly for about 12-13 seconds, which seems too long. There's about as much water filling the cistern as possible without reaching the overflow pipe - but not quite enough power!

Thanks for your help!