This is currently reproducible on version 1.14.21 for Ubuntu release.  This is not currently an option available.  

The more threads that are enabled increases the problem.  Typically over 4 threads will cause a longer than normal load time when entering the never.  If I set max-threads=0,  all cores go 90-100% when enter the nether. The load time goes up to +35 seconds 

Similarly, when only using 4 threads,  the load time is under 5 seconds.

I have not been able to reproduce this problem from a windows server.