Version 1.5 of Query Cache for Zen Cart has been released. New version reduces query count by 80% (previous version reduced query count by ‘only’ 50%). Here’s list of changes:

970 queries down to 198 queries ( v1.0 executed about 450 queries )
some performance improvements to code
includes/functions/functions_categories.php has been rewritten to use cache and prefetch data. [...]

Performance, Query Cache, Zen Cart

Update: Query Cache V1.6 has been released – read here
Query Cache is free, in memory cache designed to work with Zen Cart. It can reduce number of queries sent to database by over 80% (see charts below). Thus, Query Cache  might greatly reduce query execution time and response time of most Zen Cart stores.
You might [...]

Performance, Query Cache, screencast, Speed, Zen Cart