import_error.py

# run it on a buildout.coredev checkout, plone 5 branch
# save its output so that you have the same imports as on ``pkg_resources.py``
import os

for folder in  os.listdir('src/'):
    print("""
try:
    import {0}
except ImportError:
    print('{0} not found')
""".format(folder))

pkg_resources.py

# run it on a buildout.coredev checkout, plone 5 branch
import pkg_resources
import os

for folder in  os.listdir('src/'):

    try:
        pkg_resources.get_distribution(folder)
    except pkg_resources.DistributionNotFound:
        print('{0} not found'.format(folder))

With this two files you can roughly compare how fast/slow is checking for and ImportError rather than a DistributionNotFound.