Looking at the details:

3.8 million data store reads for 8,000 visitors seems high. Especially for the kind of data you are serving - it's not rapidly changing.

Are you using memcache at all? A quick scan of the source doesn't show any memcache imports, but I just might've missed it. (http://code.google.com/appengine/docs/python/memcache/usingm...)