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Can confirm this for a brand new 1.13.0 world. Chunks load very sporadically. I also get block lag and some blocks just won't render their texture at all( I can see the hitboxes outlined). This happens both on loading and generating chunks. I have no performance issues on 1.12.2 with vanilla mc( no optifine). One thing worth mentioning is that performance improved a lot when I limited my fps to 30 via the in-game option, so this might be some timing issue between threads and/or draw calls. I'm using linux( fedora 27) with a dedicated amd gpu and a quad core i7 that doesn't throttle so performance should be plenty.

I'm trying to load the world in 1.12.2 but it is stuck on the loading screen forever. I think it was fixed on 1.11 with the changes Jeb did to hoppers.

As I mentioned the issue is present on vanilla when comparing the intel and the amd card. Optifine suffers from it too but less. Is there any way in vanilla 1.12.1 to change the fog density so I can do more comparisons ?

Using optifine on linux( fedora 26 with 4.12 kernel, mesa 17.1.5) on an amd dedicated card( R9 m265x, Cape Verde) and an intel integrated card( hd 4600), I can see fancy fog being denser on the amd card vs the intel card( the intel one doesn't seem to do anything different on fast vs fancy) but not as dense as the screenshots on OP. Same goes for the generic fog on vanilla but I can't find the setting to change that anywhere on the video settigs. Here are the openGL Core/ES/Extension versions supported by both cards:

[headsuphigh@192 ~]$ glxinfo | grep "version"
server glx version string: 1.4
client glx version string: 1.4
GLX version: 1.4
Max core profile version: 4.5
Max compat profile version: 3.0
Max GLES1 profile version: 1.1
Max GLES[23] profile version: 3.1
OpenGL core profile version string: 4.5 (Core Profile) Mesa 17.1.5
OpenGL core profile shading language version string: 4.50
OpenGL version string: 3.0 Mesa 17.1.5
OpenGL shading language version string: 1.30
OpenGL ES profile version string: OpenGL ES 3.1 Mesa 17.1.5
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.10

You can also find every extension supported per driver on www.mesamatrix.net

Can confirm this is happening in 1.10.2. Fancy fog uses a function only available on NVIDIA drivers.

Yes that is exactly what happens, the empty space was on the chest that was on the left chunk. The workaround is to update the hopper. I also checked the rest of the chest tower and it seems like all of the hoppers had this issue

After posting this I tried deleting the region files again and regenerating that fort in both survival/creative/spectator mode a few times and it was the same.

This appeared only after I updated to 1.10. I have never experienced this in 1.9.x

seed is -3862419354477532128. It happens at random I just flied around until I saw one. It's more intense in large lava lakes. Cords from the second screenshot are 269 40 -4