Here are some suggestions, assuming you're running the latest-released version of Zen Cart:
1. Turn off Category Counts, especially if you have a large number of categories.
a. Admin->Config->My Store->Show Category Counts=false
b. Admin->Config->My Store->Show Category Counts-Admin = false
(this second one is for admin-area only when editing catalog)
2. Disable the Manufacturers sidebox if you don't need it.
Admin->Tools->Layout Boxes Controller
- find the sideboxes/manufacturers.php entry, and turn it "OFF"
3. Ensure your images are optimized for your site... specifically, use small images for thumbnails, slightly larger for product pages ("_MED" images), and large detailed images for "click to enlarge" ("_LRG") images.
Related FAQ articles: Adding