SQLite isn't (typically) slow. It's incredibly performant and used in all sorts of speed critical systems - http://www.sqlite.org/famous.html. That aside, SQLite isn't going to be a key bottleneck in most Rails applications where the less-than-speedy ActiveRecord is involved ;-)

SQLite's own site - http://www.sqlite.org/whentouse.html - notes that it is ideal for "the database engine for low to medium traffic websites (which is to say, 99.9% of all websites)" where they're considering this being under 100k requests per day.

It does depend greatly on what you're doing, but SQLite can vary from being a bit slower to somewhat faster than something like MySQL (many concurrent writes is where it's going to fall to its knees, not a common small web application scenario). It certainly isn't universally "slow." Searching Google for "sqlite is fast" brings up many good citations.