For cairns:

The 5% decrease is cumulative. So if you sanctify or defile 20 or more cairns all at once you will not receive any xp at all for additional cairns. If you keep sanctifying, you will keep adding to your total even though you get zero XP. For example, sanctify 35 cairns and you will have -75% xp. You won't ever lose XP from sanctifying or defiling the cairns, but it will take longer to get in the positive. E.g. to get back to 100% xp it will take ~175-200 minutes of waiting.

You will also notice that you receive two XP messages for cairns. One is for the portion that counts towards both quest and total, the second is for the portion that counts only towards your total xp.

- Dec

Penned by my hand on the 23rd of Artificium, in the year 71 AM.