I started playing with Twitter just a few days before it joined the Web 2.0 in-crowd. Since gaining popularity, I have noticed that Twitter website performance had been terrible, which didn’t exactly surprise me. What I didn’t know, was that Twitter’s performance problem was largely due to its use of Ruby on Rails, according to this Coding Horror post..
A friend of mine was interested in picking up Rails because it was the cool language right now. I had reservations because I suspected that all the sugar coating was bound to have a performance/scalability impact. 
[Update @ 2007-04-17T22:18-8:00] Here is a counter point.
really? I tried it and seems the performance is ok. at least for me. are you sure it’s not your bandwidth to blame?
The performance is getting better, but it was really bad those few days when Twitter first hit it big. As for the bandwidth, I would hate to think that my 3.0mbs DSL download speed is not fast enough for Twitter.
Besides, would I lie to you?