if this is a flying wing design then you need to chose an airfoil with no or low pitching moment. e.g. a reflexed section or a symmetrical.

If it's a tailed design then you are not restricted by that requirement.

IMHO the speed of your aircraft is more likely to be affected by powerplant/prop or weight issues.

Also wing aspect ratio has a big effect on total drag - especially when you are in a high lift area of flight like a tight turn. Low aspect ratio wings will be lighter for a given strength and they will roll faster (very important in combat) but will also create more induced drag than higher aspect ratio wings.

Roughly speaking a wing with aspect ratio 5 with RG15 a good sailplane airfoil has about the same drag as a wing of aspect ratio 7 with a draggy eppler symmetrical airfoil with thickest point forward at 20%.

this is of course a generalisation but its worth looking at aspect ratio when you are trying to minimise drag. However you need a model that can roll fast and turn tightly more than a low sink rate high Lift/drag wing.

Keep the weight down as far as possible and get your engine tuned to the max. If you are using a tailed design then couple elevator to ailerons for tighter turns.