If you want to alternate between 60 FPS in the menus and 25 FPS in the game, there is indeed a way to do that - you can just use Python to set the logic tick rate (the drawing and logic FPS) where you need it to be. Here’s the code for this.
from bge import logic
logic.setLogicTicRate(25) # 25 FPS for logic and drawing
logic.setPhysicsTicRate(60) # Keep the physics calculated at 60 FPS
Alter the values to make the game perform how you wish, and you can do this separately, one for the menu, and one for the actual game. However, note that altering the logic tic rate is altering the drawing tic rate, meaning that if you have an animation, it will play back slower.