Opening a Wikipedia page takes several minutes. I've initially found the bug in Luakit (https://github.com/luakit/luakit/issues/633), but I've tested a few other webkit2gtk-based browsers (Epiphany, Midori, uzbl), and the same problem occurs. Strangely enough, other Luakit users (at least 2...) have no such problem. I don't know if it's relevant, but qutebrowser (based on qt5-webkit) works fine. My OS is Arch, where the version of webkit2gtk is 2.18.5-1.
And now, after more than a month with this problem, it has gone, like that. I've tried to pinpoint what changed in recent upgrades, but I didn't find anything. I hope it stays like that.