Enabling clouds incurs a huge performance penalty

On lower-end hardware (a laptop with an integrated AMD GPU running Linux), the performance hit is much more relevant. Enabling clouds drops the frame rate to an average of 30 from 45-50.

Looks to be Works As Intended for me- obviously not a preferable fact, but probably just Minecraft working harder to render clouds.

The issue is not reproducible in Minecraft 1.7.10 or earlier, so it's likely to be a performance degradation from recent updates given that clouds have not changed visually at all.

I don't know; while I would expect clouds to incur some performance costs, it being the difference between 674 FPS and 366 FPS is a bit high.

Can confirm this, while I was building my map.
Off=180fps
On=20fps

Can no longer reproduce in 1.13-pre2.

Probably was actually fixed in 18w10d; the changelog mentions specifically "Big optimizations to how clouds are rendered". I don't recall ever experiencing this myself, but if you can consistently check it it'd be nice to test that version.

Confirmed that this was fixed in 18w10d.  In 18w10c, on a superflat world with no mobs, clouds take up 30%-45% of the gamerenderer pie chart section in the shift+F3 profiler.  In 18w10d, it's between 10% and 15%.