Right now, Twitter is suffering slowdowns. Earlier today it was down again. :(
There are an excessive number of single points of failure in the current system, and through developer error* and external circumstance we have managed to hit quite a few of them in the last week. I am sorry and embarrassed.
In particular, we mis-estimated the impact of some cache policy changes. If your site runs so hot that it can’t function without memcached, you’d better understand exactly how much buffer capacity you actually have.
We’re working on fixing it all, but it takes a long time…
* I ain’t sayin’ it was me, but I ain’t sayin’ it wasn’t.