I'm pretty sure the "optterrain" command is irrelevant if you started a new game on the latest patch. The "optterrain" was to implement and optimize terrain on worlds with old saves that weren't affected by the optimize terrain patch. If you started a game on the latest patch, then "optterrain" will do nothing.

yes at some point changes in any one area will add up and cause lag. if its an early world you are on you should enable console commands (Find Valheim in your games Library and right click on it. Select Properties to open up the options. In the General tab, enter “-console”) and preform "optterrain" command in any area you've done major landscaping in and it will attempt to optimize the changes to the new more performance friendly system.
In any area you can hit f2 and look at "active instances". this is basically all the objects locally the game has to keep track of, including terrain modifications. People can start having lag at anywhere from 10k-20k depending on their system specs, so you can use it as a rough idea of where you should stop building/landscaping in any given area before it causes you too much loss of framerate.
Also multiplayer will always be lower performance, and can be more so depending on the specs the server is being run on.