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:
This error generally means that one or more system tables cannot be found in the database.
This can occur if you've provided valid DB credentials in your configure.php files and the database itself exists but contains no tables. In this case you need to run zc_install to properly setup the database contents.
Further, this symptom most commonly occurs when moving your site to another server and not properly preserving the DB_PREFIX setting from the old site's configure.php into the new site's configure.php, thus the tablenames cannot be located because they don't match up properly. Or you need to run zc_install to create the proper tables. Or you need to import the correct matching database data, since what you've got in the database presently isn't suitable for your current DB_PREFIX configuration.
(To manually figure out the correct prefix to use, look at the database using phpMyAdmin and see what common 2-4 letter prefix starts all the tablenames. Then set that value into the DB_PREFIX definition in your configure.php file.)
The following error occurs when trying to process a payment through Paypal express near the final stages.
We are sorry for the inconvenience; however, at the present time we are unable to use PayPal to process orders from the geographic region you selected as your PayPal address. Please continue using normal checkout and select from the available payment methods to complete your order.
However, alterations added by the storeowner may behave differently, including but not limited to tracking addons, analytics scripts, tracking pixels, etc. Those are up to the storeowner to disclose.