UPDATE: I have tested and the issue is not present in 1.12.2. I have not tried other versions besides 1.12.2 and 1.16.1, however.
I am experiencing lag spikes while trying to play Minecraft. It seems to be tied to loading certain chunks. Here is a video of the issue. The FPS stays well above 120 FPS in F3, but you can see the tall, thin red spike in the frame time graph as I move back and forth in the world. In the video I'm running Sodium for Fabric but it happens in both vanilla and Optifine as well. Note also that the issue occurs more generally, not with just this specific chunk. This is just the example that I could most easily replicate. I am running Pop!_OS 20.04 (which is based on Ubuntu), and I have an i7-10875H CPU, NVIDIA RTX 2080 Super, and 64 GB DDR4 RAM. I have tried all of the standard recommendations I've seen online: restarting PC, uninstalling/reinstalling Minecraft, fiddling with video and rendering settings, closing all other applications, ensuring drivers are updated, ensuring Minecraft is using NVIDIA Graphics, installing Optifine or Sodium, allocating different amounts of RAM to Minecraft, and so on. Nothing has worked to stop the lag spikes from happening.