explain analyze reports that my queries are fast but they run very slowly

We just upgraded from 8.3 to 9.1 and we're seeing some performance
problems. When we EXPLAIN ANALYZE our queries the explain result claim
that the queries are reasonably fast but the wall clock time is way way
longer. Does anyone know why this might happen?

Like so:
db=>\timing
db=>EXPLAIN ANALYZE SELECT max(id) FROM foo WHERE blah_id = 1209123;

The plan is sensible. The estimates are sensible. The actual DB time
reads like it is very sensible. But the wall clock time is like 11 seconds
and the \timing report confirms it.

Any ideas?

Thanks!

Nik