I found a really bad compatibility issue with the mods 'WideChests', 'Merging Chests All Types', 'Merging Chests Logistic', and 'Merging Chests Unlimited'.
The game ran fine until I landed on Cerys, but after I landed on Cerys, the game lagging. The game starts to freezing for a split second, and unfreezing for a split second. Those freezes are less severe after switching view to another planet, but the stutters are still present, just every second or so.

I was not able to reproduce this.

So, I finally had some time to sit down and troubleshoot, and I found the source of the problem. The issue lies in the mod (WideChests) configuration. By default, the maximum length and height of the merged chest are set to 10x10, but you can increase this up to 42x42. With the default settings, the game runs fine, but I had to crank it up to 40x40 because I like using those really long and thin chests at train stops. So, when I have the mods I mentioned above active and Cerys active at the same time, and I'm on Cerys, I start to have those performance issues.

Ah yes, then there are thousands of chest entities in the game :) The performance improvement in 2.5.14 should fix this for you.

But that's the thing: even after starting a new game (with this 40x40 setting), the game runs fine. But if I select the option to start on Cerys in the mod menu, I have the same issue. I haven't even built a single structure. I have nothing, and I'm just running around, and the game is lagging. Also, I noticed that when I'm running around, the lag gets worse and worse. As I start revealing the map, the performance goes from bad at the start to unplayable after revealing the entire moon's map.

Is that still the case on Cerys v2.5.14?

Just updated to v2.5.14 and now everything looks fine. Thanks for the help.