How to enable igzip multi-threaded run to improve compression throughput, -T parameter does not work

I download the isa-l's soure code，after compiling，open the directory programs/.libs ,find two bin files, igzip and lt-igzip, running as

**first**: time ./igzip -z /raid0/data/fq/SRR9613620_1.fq -o /raid0/data/fq/9613620_00.gz

real 0m41.708s
user 0m36.434s
sys 0m5.177s

**second**: time ./igzip -T 10 -z /raid0/data/fq/SRR9613620_1.fq -o /raid0/data/fq/9613620_000.gz

real 0m40.680s
user 0m36.335s
sys 0m4.249s

The time of first and second is nearly, mean that -T parameter does not work.

Thank you for your help.