PrestaShop: the mistakes that slow down your online store

On PrestaShop, speed has a direct impact on sales: every extra second of loading time drives away part of your visitors before they have even seen the catalog, and Google takes it into account in its rankings. Here are the mistakes I find most often when working on existing stores, in the order I check them, and how to fix them.

First, measure

Before changing anything, you need to know where the time goes. Two measurements are enough to start with:

• Server response time (TTFB): how long the server takes to send the page back. Above roughly half a second on a category page, the problem is on the server side (modules, database, hosting), not the images.
• The PageSpeed Insights report for a product page and a category page, on mobile: it shows the weight of images, scripts and fonts.

On a copy of the store (never in production), PrestaShop’s profiling mode details the time spent by each module and each SQL query. It is enabled in config/defines.inc.php:

define('_PS_DEBUG_PROFILING_', true);

1. Debug mode on, or caching off, in production

This is the simplest mistake to fix and one of the most common, often after a migration or a technical intervention. In Advanced Parameters > Performance, check that:

• debug mode is off;
• Smarty template compilation is set to Never recompile template files (or at least Recompile templates if the files have been updated), never to Force compilation;
• the Smarty cache is on;
• CSS and JavaScript minification and concatenation (CCC) are on, then tested page by page: some badly written modules do not support it.

2. Too many modules installed “just in case”

Every module hooked into the theme’s display points adds code that runs on every page load, even if its feature is almost never used. The most expensive ones hook into the header or footer, because they run on every page.

• List the active modules and ask, for each one, whether it was used this month.
• Actually uninstall the ones you do not use: a module that is only disabled often leaves its tables, and sometimes its scheduled tasks, behind.
• In Design > Positions, check what is hooked into displayHeader and displayFooter.

3. An overloaded theme

Premium themes ship with dozens of options designed to suit everyone: sliders, animations, carousels, several fonts, sometimes several JavaScript libraries that do the same thing. Turning off what your store does not use makes every page noticeably lighter. If the theme was edited directly, without a child theme, every update becomes risky: that is often the time to clean up those changes.

4. Product images that are too heavy

A catalog with hundreds of products and photos uploaded as supplied by vendors (often several megabytes each) weighs down every category page. Three actions:

• resize and compress images on import, before they reach the catalog;
• in Design > Image Settings, check that the generated sizes match what the theme actually displays, then regenerate the thumbnails;
• serve a modern format such as WebP: recent PrestaShop versions can generate it, otherwise a module or a CDN can handle it.

5. A database that grows without anyone noticing

Some tables grow silently: connection statistics (ps_connections, ps_guest), pages not found (ps_pagenotfound), logs (ps_log), abandoned carts. After a few years they can outweigh the catalog and slow down both the back office and the storefront. To find the largest ones:

SELECT table_name, ROUND((data_length + index_length) / 1024 / 1024, 1) AS size_mb
FROM information_schema.tables
WHERE table_schema = DATABASE()
ORDER BY size_mb DESC
LIMIT 10;

Clean-up must be done carefully, after a verified backup (see the backup rule almost nobody follows), and by turning off the statistics modules you do not use.

6. Hosting sized for a brochure website

PrestaShop needs more resources than a brochure website, especially with a large catalog. Points to check with your host:

• a recent PHP version supported by your PrestaShop version, with OPcache enabled;
• enough PHP memory for the back office and imports;
• a CDN such as Cloudflare for images and static files, which takes load off the server during traffic peaks.

The checklist

1. Measure server response time and the mobile score of a product page and a category page.
2. Debug mode off, Smarty cache and CCC on.
3. Unused modules uninstalled, header and footer hooks checked.
4. Unused theme options turned off.
5. Images resized, thumbnails regenerated, WebP format.
6. Statistics and log tables monitored and cleaned after a backup.
7. Recent PHP with OPcache, CDN for static files.
8. Measure again, to check that each fix had an effect.

Going further: my expertise: Freelance PrestaShop developer · read also: Migrating your PrestaShop store without losing your search rankings