Quicker,
The first number is the length of the prop if measured from tip to tip. The second number is the pitch. The only way I know how to explain that is to say: In a perfect substance, say jello, one complete revolution of the prop would go through 7" of jello, if we are using the 9x7 for the example. Therefore a 8x6 would/should slow your airplane down as far as top speed is concerned. But I suspect the 9x7 is hampering (too big) the LA25 and not letting it rev to it's full potential. If you want speed then go with the larger pitch (second number) and smaller diameter prop, like a 8x7. You will have to try a few different props to find the one you like, good thing the little ones are cheap. Either way I'm sure you will like it a lot more when you get the right prop for the engine. Prop selection is dependent on what kind of flying you want to do, if you like to go fast use small diameter, big pitch. If you like to do fun fly or do aerobatics, go big diameter, small pitch. Big diameter, small pitch props will make the plane fly slower but have more power, like a John Deere. The opposite is true for the small diameter, large pitch props. Maybe some one else will explain it better than I can.
Tim