Sometimes the game simply runs faster than others. I don't mean frame rates or ping times by that, either. At its most extreme, sometimes everything only happens at about 60% of the normal speed. That's running speed, attack speed, skill cooldowns, everything. Mobs move and attack slower, too, so it's not a huge gameplay disadvantage, though it is annoying.
Well, not quite everything: buff durations still last the normal length. So if you have a buff with a 40 second duration and a 30 second cooldown and the server is only running at 60% speed, the buff still lasts 40 seconds, but the cooldown is now 50 seconds so you can't maintain it indefinitely.
Usually it's not 60% speed. That seems to only happen right after patches. But a lot of times, it's about 90% speed. Today, it seems to be around 85% speed. And the speed at which the server runs seems to stay the same all day, not bounce around constantly.
I'm not sure if it's a bug or an intentional way for the server to cope with heavy loads. It would be bizarre as a bug, though. And if it's a way to cope with heavy loads, why is it still present at far off peak times (e.g., weekday mornings) when there is a light load on the servers?