Significant Lag increasing as singleplayer game progresses...

Starting my game, no lag, up to 60 FPS, hardware not taxed. (3.0 Ghz i5 CPU, 1080 OC GPU running top usage of 60% and 20%, respectively). However, the longer I play the more my game lags, going into single digit FPS after <hour, and still only using fraction of system resources. Restarting game resets lag to zero, but increasing exponentially.
Additionally, note following Valheim system usage while lagging into 1 FPS (approx max usage in each category):
Memory: ~ 4Gb usage of 26Gb
Disk: 0 to 2% of Solid State hard drive
Network: 0%
I meet or exceed all recommended system specs listed on Steam. I play with almost all graphics settings at minimum, excepting view distance at high to max and vegetation on medium. Even tried turning down all graphics and only minimal lag decrease can be discerned. I do not have issues with other high-end games on my very stable computer system. Note attached system usage image as I just sit in game doing nothing. 19fps is rare high after 2+ hours of play.