I maybe encounter 1 battle every minute or so, as opposed to 1 every few tiles in other fests. Is this a bug or intentional?

Umbra's Veil has a very slightly lower encounter rate compared to say, Esior's Cove from the Summer Festival. This is due to its size being significantly larger. We felt it would make things a bit less frustrating to walk from Named to Named with the slightly less encounter rate.

There is also a safe zone when you first enter Umbra's Veil. So if you are trying to find battles near the entrance, that might explain it.

Otherwise though, no, I wouldn't expect there to really be any noticeable difference. The difference is very small - like maybe requiring an extra tile or two every couple encounters - small. If you're spamming movement to go places, this difference would be a fraction of a second.

Ah, thanks for the clarification~ I didn't think I was near the entrance but I moved and now it's a little better xD