This is due to an underlying library using the maximum number of available threads to execute a code. This causes drops in performance. When declaring the previous variable, you will only allow one thread per process, thus restoring expected performance.
> Note that trying low values such as `OMP_NUM_THREADS=2` or `OMP_NUM_THREADS=4` may give you better performance.
Other factors that might harm performance:
* Slower hard drives
* Slower hard drives (be sure to use `/ssd` disk space for you IOs)
