Increasing the time between spawns would likely help, rather than reducing slots, or perhaps in conjunction to reducing slots. But I'd try just increasing the time for the spawn waves first.
I'd also move the flags out of the buildings, putting the allied flag in front of the town hall, and the axis in front of the church. Since even getting to the flags seems to be such a grind, putting them in a more exposed and open area, could really speed things up in a good way.