How to enable precombined textures:
1. Go to your Fallout4.ini in C:\Users\%username%\Documents\My games\Fallout4
2. Search for bUseCombinedObjects and set the value to = 1
3. Search for bUsePreCreatedSCOL and set the value to = 1


If you want to learn more about precombined textures I highly recommend watching Red Rocket TV's videos:

1. Fallout 4 Let's Play Prologue 2: Performance and Optimization
https://www.youtube.com/watch?v=yQX-F_zBGGM

2. Fallout 4 Creation Kit Tutorial: Regenerating precombined meshes and previs data
https://www.youtube.com/watch?v=hPENZq2ub1c