When I played Subnautica on a HDD during Early Access the pop-in was unbearably bad, but optimizations during development fixed the worst of it. The removal of digging and terrain modification alone basically solved pop-in for most areas - the mushroom forest was still pretty bad, but they also patched that later in development.
Initial load-in will likely take a while though. It took a few minutes to get into the game from the main menu the last time I had it installed to a HDD.
When I played Subnautica on a HDD during Early Access the pop-in was unbearably bad, but optimizations during development fixed the worst of it. The removal of digging and terrain modification alone basically solved pop-in for most areas - the mushroom forest was still pretty bad, but they also patched that later in development.
Initial load-in will likely take a while though. It took a few minutes to get into the game from the main menu the last time I had it installed to a HDD.