not sure bat speed will be an issue. Personally I think trying to work the ball off the back foot behind square on the leg side doesn't sound like an easy shot!
It seems as if on both occasions, you were batting with a 7-2 off side field in front of you, on that basis the bowler will be trying to bowl the ball on off stump and move it away from you or bowl in the "corridor". Given that set up trying to work the ball behind square on the leg side seems a low percentage shot. In your shoes I would be trying to work the ball in front of square, through mid on, which would be an easier shot, given the set up.
The way the field is set should tell you what the bowler is trying to do and where they are trying to bowl at you, using that information and knowing where your main scoring areas off that type of bowler will make life easier for you.