Delay in loading webpages

Since a couple of days, my machine experiences a delay when loading webpages. Rather than starting loading, the browser sits there for several seconds (around 5 or 10 seconds), seemingly doing nothing. Chromium says "Resolving host..." in its status bar while waiting. Firefox says "Looking up [domain]" and after a few seconds "Performing a TLS handshake to [domain]..." before the page finally loads. The loading process itself is not slow; this only happens on opening, not during loading of data. An online internet speed test also reveals that the speed is the same as normal (47 Mbps down, 56 Mbps up). And another computer in the same network (running a different OS) does not have the same problem. Does anybody know what I can do to check what is wrong? I'm not running out of disk space. I cleared out my browsers' caches, but that hasn't helped. And now I don't know, e.g. which log file(s) to look in. I found several things to check when I searched, such as [Ubuntu 18.04 slow after 2019 update](https://askubuntu.com/q/1138691/61377) , but none that address this particular problem. This is with Debian 9.12, KDE.