The Bug

Having noise_caves, found in noise settings, enabled increases generation time by a lot.
This can be seen as continuation of MC-217509 which is now fixed but didn't fix all issues noted in the report. my fault, one bugreport shouldn't contain multiple issues.
To note that in MC-217509 noise_caves weren't the major slowdown, which they are now.

I got this data with a script that generates each custom world settings file 5 times and outputs the time it takes. The raw data from each time generating can be found in 21w42a-results.txt

Reproduce + Observed Result

Generate and compare the time it takes to generate a world with each custom world config.

21w42a-everything_disabled.json
average: 25.438678503036 seconds
(base)

21w42a-everything_enabled.json
average: 73.39190864563 seconds
+188.505% increase (x2.89)

21w42a-noise_caves_disabled.json (everything enabled except noise_caves)
average: 35.724261951447 seconds
+40.4329% increase (x1.40)

21w42a-noise_caves_enabled.json
average: 61.800010824203 seconds
+142.937% increase (x2.43)

As you can see in the data, generating the custom world that enables noise_caves takes a lot longer than with it disabled.

Expected Result

Noise cave generation is a feature that is expected to take some resources and time, but the amount of time it takes right now is a lot and would preferably be a lot less.