If you try to hack a portal the 2nd time in a short time, you get a message, "The portal is hot" and has a 300 second delay. But it seems you always start with a 300 second delay. This afternoon, I was at a bus stop with 2 portals in range. I hacked one and recharged it, then I did the same to the other. I tried hacking the first one again, and I got a 300 second delay, while it clearly was 1 to 2 minutes later then I previously "re-hacked" a portal.
So I was wondering if this delay was just for me and if it was like a penalty for hacking the portal in such a short time. My philosophy is the hidden cooldown might just be two minutes and when "rehacking" on 1:59 you get a 300 second delay on top of it.