It could be graphics, the look is intentional and really only the textures are low grade - under the hood Valheim can still burn heavy GPUs.

Auto save is every 30 minutes, there is no options to adjust it. The more explored your world the longer a save takes, but there's a warning before it does it. The mistlands introduced save-like freezes every 10 minutes seemingly unconnected to autosave, which has been complained about a bit.

But yeah if it's more frequent it's probably something else again.