Since different uzbl instances do not share their cache, when you query the same page in 2 different uzbl instances, the query is done twice (If you think in the unix way, caching is not the work of a web browser). You can solve this improve the speed of uzbl by installing a local caching proxy.

For example, you can install squid, in combination with privoxy for adblocking. There is a nice example of how to do it on the privoxy website : http://www.privoxy.org/user-manual/config.html#FORWARDING

Polipo is a lighter weight alternative to squid thats designed for single users or small groups making it ideal for use with uzbl http://www.pps.jussieu.fr/~jch/software/polipo/