What is considered a long execution time?

YouTube's target page rendering time is < 100ms (Video here @7:00).

Your bottleneck is probably DB queries - try using

EXPLAIN select * from x...

to see if you can add indexes that will speed up your queries.

edit the link above has died. High Scalability did a feature on YouTube that used that video as its primary source, so it may be of some interest: http://highscalability.com/youtube-architecture

Hum... I'm not sure an absolute value is quite fair here. It really depends on the hardware... When I develop locally, my developper machine runs something like 5-10 times slower than the actuel server. So if we take an absolute value, the "acceptable" range would vary depending on the hardware.

Well, generally I try to keep things below 100 ms. If the server load time is higher, I'll trace the execution and try to figure out what's wrong. I have to say most of the time, the databse (hence the queries) are the bottleneck. A real work on that is really important.

Seems a bit high to me.

For reference, my framework that I've created is getting execution times as low as .0028 and as high as .0340 seconds. On average, each page typically has between 11 - 18 SQL queries.

However, keep in mind that this is a highly optimized framework, taking advantage of caching, very careful coding of queries, and autoloading. Try implementing those tactics, and you should see a big improvement.