V-sync is a system used by Fallout 4 and almost every other game to match the game’s FPS with your monitor’s refresh rate. Most games have an option in their setting to disable it, but F4 does not. It is not 100% clear to me why it makes you lag. The lag only seems to happed indoors, when looking at the ground or another specific part. Fallout just has a hard time with v-sync so we will disable it along with a few other tweaks to get the game running smoothly.

HOW I RECOMMEND: There are a few different ways of doing this. The way I recommend is to download the Fallout 4 Configuration Tool - By Bilago. http://www.nexusmods.com/fallout4/mods/102/?
This is an alternate launcher for F4, it gives you a huge amount of other config options, along with the option of disabling v-sync.

Once the launcher is installed (just follow the directions on the nexus page) navigate to the performance tab, and set v-sync to 0, and save. This disables F4’s v-sync letting the FPS run wild. \
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ALTERNATE/MORE INVOLVED METHODE: This is editing the .ini files of F4 and manually disabling v-sync. The first step to this is to find F4’s .ini files, usually they are located in the documents section of your pc. Here is the location of mine:

The next step is to open Fallout4Prefs.ini,find the line iPresentInterval=1 and change it from 1 to 0, save and exit.
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Chose which ever method is best for you and follow it, if you have done things correctly, v-sync should now be disabled. To make sure, open F4 and look at your FPS at the title screen, it should be some outrageous number. If so, then move on to step 2.

This step is VITAL if you want a playable game. Since we turned off v-sync the game’s FPS is now as high as your system can push it. You might think “Hey! Big FPS is good right?” Wrong. Fallout’s engine makes the game run fast depending on what your FPS is, so if you are in a small room getting 120 FPS, your game will run twice as fast (meaning you will run, shoot, look twice as fast). Now we will disable this. I am going to show you how to do it with AMP cards, I assume GTX cards are roughly the same process.

Step 1 is to right click on your desktop and open AMD Radeon Settings.
Step 2 Click on the gaming tab
Step 3 Click on global settings
Step 4 Turn on Frame Rate Target Control and set the FPS limit to 60 or above (I use 70)
Step 5 Close ARS and open F4
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If the settings are in place correctly then F4 should be running correctly now with no v-sync issues. To test go to some of the problem areas where you had bad FPS drops. Note this will not improve FPS if your system simply can’t run the game, it will only help where v-sync was causing problems.

Leave a rating or comment if you liked the guide, ask questions and I will try my best to help ya out.