[Waiting for user feedback] My website is slow.

Thank you for providing access to a staging site!

The site has many available updates. Please always run all the updates to confirm if the issue persists once everything is up-to-date. I already ran some updates, but there are also some plugins that can not be updated, as they are premium versions.

For example, "WooCommerce Additional Variation Images" has not been updated for a long time.

You will find updates for WPML at Plugins > Add new on the "Commercial" tab after clicking "Check for Updates".

Note that you need to make sure to register WPML first at:
https://wpml.org/account/sites/

Then copy the site key and enter to at Plugins > Add new on the "Commercial" in the top right corner. After that, run all updates.

I have already enabled WP DEBUG on the staging to check for errors.

Let me know once you are done with this task.

Based on your screenshot, there should be an unexpectedly high load time when calling:
hidden link

Calling this URL on your site takes about 3 seconds and on second load, 2.5 seconds.

If I disable WPML, it takes 2 - 2.5 seconds. An impact of 0.5-1.0 seconds in load time can be considered normal when using WPML, due to its complexity.

Note:
You are using a plugin called "Disable REST API". Please note that the WP REST API is mandatory for WPML to function properly, and it should not be deactivated.

Source:
https://wpml.org/documentation/support/rest-api-dependencies/

I installed the Query Monitor Plugin on your site, but I can not identify any load times that are far beyond the expected values.

Example "Homepage":

With WPML, it takes 3.6 seconds to load the Homepage.

If I try testing the homepage without WPML, I get an error:

Uncaught Error: Undefined constant "ICL_LANGUAGE_CODE"
in /www/thephonelab_868/public/wp-content/themes/thephonelab/includes/actions.php on line 312

Call stack:

lf_redirect_when_cart_is_empty('')
wp-includes/class-wp-hook.php:324
WP_Hook::apply_filters(NULL, array)
wp-includes/class-wp-hook.php:348
WP_Hook::do_action(array)
wp-includes/plugin.php:517
do_action('template_redirect')
wp-includes/template-loader.php:13
require_once('/www/thephonelab_868.../template-loader.php')
wp-blog-header.php:19
require('/www/thephonelab_868/public/wp-blog-header.php')
index.php:17

This happens if you use WPML-related code without checking if WPML is currently active on the site. If you use code like that, I suggest you wrap it into something like:

if (class_exists('sitepress')){
  // your code
}

Also, please verify if the issue on the site might be theme-related, as this theme is using custom code.

I then switched to the Twenty Twenty One Theme for testing, and the Homepage still needs 2.5 seconds to load entirely, even without running WPML. Enabling WPML at this point leads to a load time of 3.4 seconds on first load, 3.0 seconds on second load, and 2.5 seconds on third load.

This means there is an impact of up to 1 second when running WPML, but this is only the case when loading the page for the first time, as after that, the caching will begin to operate.

If this is the issue that you would like to report, then I can sadly not consider this a performance issue, as the impact is currently expected when running WPML.

If it is a different issue that you would like to report, please name me another URL as an example of an unexpectedly high load time caused by WPML.

ON your dev site, the REST API is completely disabled. I only mentioned this, as you might run into issues with your translations if you disable the REST API.

About the performance issues:
I could not yet confirm any issue where the load time is impacted by WPML in an unexpected way.

Most pages load in 1.6-3.0 seconds, testing with a default theme.

I can only confirm slightly higher load times up to 4 seconds once the custom theme is enabled, which means this difference seems to be theme-related.

Do you know if there may be one specific page or post on your site with an unexpectedly high loading time, or maybe between 5-10 seconds?

Also, please ask the developer of your theme to solve the errors that occur in the theme once WPML is disabled. (Developer is calling WPML-related code without checking if WPML is active)

Yes, there can be an impact on the backend when running WPML, specifically when saving original content, as WPML then needs to sync settings and translation statuses between all active languages.

The more active the language, the longer the load time on the backend, but it should not be bigger than 7-10 Seconds when saving a post or page on the WordPress Editor.

Currently, I see your backend behaves slow.

There are at least three errors:

This one relates to "BCM Duplicate Menu" and should be reported to their support team:

Functie _load_textdomain_just_in_time is onjuist aangeroepen. Translation loading for the bcm-duplicate-menu domain was triggered too early. This is usually an indicator for some code in the plugin or theme running too early. Translations should be loaded at the init action or later. (Dit bericht is toegevoegd in versie 6.7.0.)	

There are two other errors that should be reported to "Smart SEO Data CSV Import/Export":

Functie register_rest_route is onjuist aangeroepen. In de REST API route definitie voor smartseo/v1/export ontbreekt het vereiste permission_callback argument. Voor REST API routes die bedoeld zijn openbaar te zijn, gebruik __return_true als de toestemming callback. (Dit bericht is toegevoegd in versie 5.5.0.)	

Functie register_rest_route is onjuist aangeroepen. In de REST API route definitie voor smartseo/v1/import ontbreekt het vereiste permission_callback argument. Voor REST API routes die bedoeld zijn openbaar te zijn, gebruik __return_true als de toestemming callback. (Dit bericht is toegevoegd in versie 5.5.0.)

If I disable those two plugins, the load time on the backend decreases from 13 seconds to 4 seconds.

I can still see that saving a page on the WordPress Editor still takes around 12 seconds, which is quite high for a site with only two active languages.

Anyhow, if I disable WPML, it still takes around 9 seconds to take the page on the WordPress Editor, which means the impact by WPML is only 2-3 seconds.