To make a mob spawner efficient, make sure there's a 7x7x7 area around the spawner for max rates. Make path ways of water stream so the mobs are out of the spawning area [Which is a 5x5x5 area around spawner] and won't get stuck in the area. Creating traps like crushers or fall traps also makes it more efficient!
Thank you for telling that it is 7x7x7. It really helps. I just created a huge area that is too large.
To make it more efficient is place the grinder near a chunk border and have all the mobs moved into the other chunk maximizing the spawn rate of the spawner.
