For about 5 to 6 weeks, I used base game plus Kingsport Horror, with the occasional smaller expansion. We scored a victory by seals every time, which is somewhat of an indictment of the expansion in itself.
To the question, though, does Kingsport Horror prolong the game. It does seem that gate bursts tend to slow the game down, while the double-doom-token Mythos cards tend to speed it up. The last game we played before I removed Kingsport featured three gate bursts toward the end of the game. It added about 45 minutes to the game. And a tiny bit of tension. But we scored yet another victory by seals.
We also had a rift open for the first time, as a result of a Mythos card, and it was underwhelming. It made me wonder why we always stationed an investigator in Kingsport for rift duty, which Dam described in another thread as lame/boring/thankless. Perhaps the reason was the menacing description of open rifts in the KH rulebook. So Mageith's statement that ignoring Kingsport all together is quite viable appears to be true.
But what does that say of the expansion itself? It just seems to water down (pun, unfortunately, intended) the AH experience. I'll use investigator's, GOOS, investigator cards, heralds, but that's about it.
I think the fundamental problem of this expansion is that none of its locations are unstable. So investigators go to Kingsport out of a sense of obligation to prevent something lame from happening as opposed to accomplishing something that will increase their chance of victory. Plus, open gates spewing out monsters are what generates AH's tension.