Yup, I rolled my own. I actually started work on this quite a while ago, but I put it aside for weeks at a time more than once, and it has also gone through one and a half complete rewrites. It makes a number of changes (improvements, in my opinion) from standard Futaba-inspired "rules," such as the email field, image files renamed by timecode, using Shift JIS (instead of Unicode), and the one-image-per-post rule, so that probably means that a lot of people won't like it. But oh well.
Please play with it, break it (but not so much my host gets pissed at me), whatever... Let me know what you think. I think at this moment it's "feature locked," as in I don't want to add any new features between now and the first public code release ('cept maybe sage-ing, which is already halfway implemented), merely squash bugs and oddities.
What you see here is its default skin. I actually posted some screenshots of it in http://nub.iichan.net/b/res/10314.html a couple of days ago, just to see folks' reaction... there wasn't one. (I was expecting a "WHAT IN HADES' NAME ARE YOU DOING, HEATHEN" flaming... mebbe I've overestimated the conservative temperaments of image board freaks.) I have also hobbled together a Futaba-ish skin basically by converting Wakaba's own futaba_style.pl; I'll take some screenshots and put 'em up in that thread.
I don't want to reply to all of your stuff right now (some stuff I see your point, some I disagree with), mainly because I want to see what others have to say as well, but I will touch on these two things:
One: The pages aren't always dynamically built. The site uses the Smarty templating engine http://smarty.php.net , which has a pretty serious caching system built in. So while it does run a PHP script for every page hit, most of the time all the script will be doing is saying "Hmm, has this page been cached already? Yes? Then here's the page. Ta!" Even when it does have to build a page, Smarty pre-parses and "compiles" all the templates into a PHP script, so it only ever has to parse the template once (unless you flush the cache, which should in theory delete the compiled templates too but that seems to be buggy at the moment, or you have what I call "Template Testing Mode" on).
Two: You think my captcha's easy to OCR? Potkettleblack, brotha. :P http://sam.zoy.org/pwntcha/ (scroll down to "Pichan." I tried emailing the guy correcting him about that, but he seems to have ignored me.) Anyway, I will admit to sacrificing hackability for code efficiency (it just uses PHP's built-in fonts, for example); even as it is, I think the captcha code could stand to be simplified a bit.
The overhead doesn't come from building the template (unless you use a REALLY bad templating engine) - the real overhead is hitting the database over and over again. The templating code only gets to work after you've hit the database, so no amount of caching in there will save you. It's up to you to be smart and avoid database hits.
Also, I notice he lists the captcha under those not broken. He may think it is "apparently weak", but it is also running on with some fairly easy settings. It can easily have the distortion turned up as needed.