Before fixing the problem, you need to understand it. FIFA 15 is a 32-bit application. On a system with 4GB RAM, Windows reserves about 1–1.5GB for kernel operations and background processes. This leaves roughly 2.5–3GB for all applications.
Select Lock to 60 FPS or Lock to 30 FPS ; locking helps prevent sudden stutters. fifa 15 lag fix 4gb ram