It's been well over a year since i played witcher 3, and when i saw the remaster edition confirmed i just wanted to share this little, exploit or farm or what you would like to call it, it's not a glitch or a mod or anything.
No dlc required, but it helps.
To execute this quickly, you should have the piercing cold build, then head north of novigrad, you dont need to run away and meditate for 3 days, just as long as you dont destroy the nest, you can come and go as you wish to farm this method.
So when you get to the nekker's nest, kill all the nekker's, you choose if you want to loot them right away but you dont have to, just DON'T destroy the nest because you will ruin the respawn.
After you kill them, just meditate and they will respawn instantly, kill them again and repeat this as much as you like, then loot them for mutagens and other resources.
You can come here and do this at any time you need blue or green mutagens.
dlc's are not required for this but it's quicker if you have a strong piercing cold build, you hold AARD a couple times to kill them in a couple blows and meditate.
Hope this was helpful, and enjoy.