Brian Hayden found performance issue that was reported at astropy/astropy#10548 .
Will git bisect identify the offending commit? Reportedly, it was not an issue in 3.2.x but an issue in 4.0.x.
Test script (runme.py) can be something like this and will be in the root directory of the repository checkout:
import time as _time
import numpy as np
from astropy import table