Suggestions:

Test with Firefox and a Chrome-related browser and another browser to see if slowness follows any specific browser.

If all browsers have the same delay, then try a desktop web browser, and enable "debugging" (on Firefox "web developer tools" and Chromium "developer tools"; for both of these the key "F12" opens up some web page/session debugging detail options.) Then, visit your web mail pages, and look at the "developer tool" options showing "network" (you may need to reload your webmail pages that are running slow after you select the network info in your browser.) It is a good idea to check your browsers for upgrades and restarting them before relying on debugging from each. Also, empty cache for each website server name providing webmail service.

Do you see any HTTP Status codes which indicate server timeout for some URL? Those are likely good leads to follow. Maybe a request to http instead of https, and there is no http service, leading to connection timeout?

Pay attention to how long your client reports each request from the web server, and see if the long delay always starts with the same web-server script/file on each browser. If long delays are reliably in the same server script/content across multiple browsers, then that script is something to investigate in your web server logs and php logs if script is php-based and you have php logging enabled. Follow the usual process for debugging PHP issues.

Also, look for "console error" message in your web browsers page debugger, especially errors or warnings related to security. There are many kinds of headers send by servers which can be used by clients to alter, or deny content, or cause delays. Maybe one web browser has a different configuration, or is sending TLS certs differently, or incorrectly, requiring an extra check to validate cert chain for one site, but not another.

Usually, if other imap clients are quick when using either imap server, but webmail web clients see a delay, there is a problem in the web server service libs/bins, or config, or php or webmail config. There are less common and weird causes like network issues specific to the web services port includng routing , DNS, firewall... or outside of networking, php config diffs, bin/library deps for any applications being used, or no reboot after major library upgrade or kernel upgrade, etc.

I am not a developer for RoundCube. Good luck!

On 2025-03-01 13:31, Marc wrote:

I have two roundcube web interfaces running on the same webserver. One of them has a slow responding UI, I already compared the config files. One is connected to a different mail server, but a regular imap client is just fast with that server.

How should I go about debugging this?