Time reloading is less than 2 seconds. The people usually stop when they fell like it (Aurora), or when they encounter resistance (everyone else).
2 second pause is not enough time to flee or take action against a shooter. If it was, the Virginia tech shooter should have been taken down during one of the many times he reloaded his handguns (he brought more than a dozen spare magazines with him.)
Pump action firearms have been used in mass shootings, as have bolt-action rifles, see the Cumbria shootings in England 2010; guy killed 12 with a bold action rifle and double barrel shotgun.
If you think the line in the Constitution is outdated and inapplicable, feel free to contact your elected officials and ask for a bill to be introduced to amend the Constitution to be more in line with what you think it should mean now.