Slow download: what causes it?

My internet connection has become very slow over the last few days, at least when I'm browsing or downloading. ping gives reasonable results (ca. 26 ms) but web pages take a long time to download. It's especially a problem when I'm logging in somewhere; on protonmail I actually got timed out repeatedly during the process. Is this just the Christmas rush clogging up the phone network or could there be another reason?

Edit: I just tried pinging protonmail and got much longer times, ca.39 ms. 26 ms is my usual test with the Google dns server 8.8.8.8. So maybe it isn't my local connection, it's something in the internet itself.