Hi @asdfqwerzxcv
Sounds to me like maybe your computer isn’t powerful enough to handle your project?
What are your computer specs?
What size are your files? 1080p? 2160p?
What frame rate is your project set at? 30fps? 60fps?
Have you tried Proxy editing and/or Preview Scaling?
Instead of one 15-minute project, have you tried to divise it in 5-minute long projects?