GNU Parallel has changed in the past 7 years. So today it can do it:

This example shows that more blocks are given to process 11 and 10 than process 4 and 5 because 4 and 5 read slower:

seq 10000000 |
  parallel -j8 --tag --roundrobin --pipe --block 1000 'pv -qL {}00000 | wc' ::: 11 4 5 6 9 8 7 10

GNU Parallel has changed in the past 7 years. So today it can do it:

This example shows that more blocks are given to process 11 and 10 than process 4 and 5 because 4 and 5 read slower:

seq 1000000 |
  parallel -j8 --tag --roundrobin --pipe --block 1k 'pv -qL {}0000 | wc' ::: 11 4 5 6 9 8 7 10