What reasons are there for my cavalry one day suddenly marching at a snails pace?

I also faced this exact problem. So my answer is based on experience and I'm not 100% sure about this.

After each round of Clash of Provinces (CoP), we have one week of CoP Warm-ups. You may have noticed that during warm-up week, we have a day off on Monday with no daylong CoP event running (only hourly events). So the week starts on Tuesday with Building event. As we don't have a CoP event on Monday, meaning no zone commemoration bonus point or acceleration for that day. So our march speed for killing monsters is way way slower than other days with CoP events running.