This isn't as much of a guide as it is a simple patch I made for the game.

I made a quick mod to Final Fantasy IV to make the in-battle speed 25fps instead of the default 15.
The reason I chose 25 rather than 30 is because (as of yet) there's no way I've found to increase FPS while at the same time maintain the battle speed, so 25fps is a bit of a compromise.
The only thing the patch does is change a hexidecimal value from 0F (15) to 19 (25).
I hope you like it :)

This video is of the regular FFIV, but the patch does the exact same thing (since they're basically running on the same engine)

Note: The pre-build binary does not include the autobattle FPS increase, just the regular one
Download this file and replace the original file inside the game's folder:
25fps exe[1drv.ms]
The original file is usually located in:
"Steam/SteamApps/Common/FINAL FANTASY IV THE AFTER YEARS/FF4A.exe"

• Download a hex editor of your choice.
  
• Change these values in the following locations (FF4A.exe)
  - 0009BAA8 (battle fps, 0F by default, change to 19 if you want 25fps)
  - 0009BACB (autobattle fps, 1E default, change to 3C if you want 60fps)
  
• Save the file

Changes
09 Oct 2022: Updated download link to use discord instead of google drive
11 May 2018: Added autobattle FPS location, courtesy of f.dai