If you're having an instance related problem and any of the following are applicable:

• Your world was created and played upon near the steam launch in early 2021
  
• You downloaded a premade world that was potentially made near the steam launch
  
• You've used any mods to mass change terrain (Any time) Every mod that does terrain changes via the console uses the original terrain save state.

you might have a terrain problem. Enabling the console and typing will solve the vast majority of legitimate terrain instance problems. This only does optimization near the player so you'll need to do it at every place edits were done.

If you have instance problems and the above doesn't apply to your issues it will be either

• You've built way too much in a single area. This includes tamed animals and crops.
  
• You've got a lot of dropped loot around that has not been picked up.
  
• You've played around with dev commands and spawned in a bunch of stuff that may or may not be visible like spawner points. Spawning excessive amounts of fish or fish spawners have been known to cause problems for some users while playing with the devcommands.

If none of these issues are related to your issue it's most likely not an instance problem.

If there is no problem and you just wanted a way to reset the terrain to original state: there is a mod for that that uses the hoe and a click while holding left alt. Despite the page not listing it as a requirement, it does require BepInEx, which is mentioned at the bottom of the description page.
https://www.nexusmods.com/valheim/mods/1113