How does the "portal particle screens" improvement work?

I'm playing as Vaulters, and I'm having difficulty parsing what the hell the "portal particle screens" system improvement does:

Specifically:

1. What is "construction effects"? Does it mean that during construction, my empire has reduced science? What if I'm building more than one of these at once? Does this penalty go away when construction is complete?
2. What does "per constructed aperture" mean? I'm guessing per portal, since portals have in their description "+1 Vaulter aperture on star system". Is there anything else which is an "aperture"?
3. How does the science bonus work? Does it mean the system's science output improves by (1 science per portal)%?
4. What does it mean "not affected by additional improvements, not cumulative"? I'm guessing the former is that portals built after that improvement is built do not count, but what does the latter mean?
5. And finally, a bit more high level: even if I have like 20 portals, how can +20% science bonus on a single system be worth -66% science on the entire empire for 15 turns?