This is a set of mechanics to add some significance to treasure and to require some down time to assimilate raw experience before it can be used to gain levels. The detailed reasoning and rules are in the attached pdf. The basic rules are:

• Characters have both a raw experience value and a refined experience value.
• New experience gained is added to the character’s raw experience total.
• Experience is moved from the raw experience total to the refined experience total in small amounts after long rests.
• The refined experience total is the value used to gain levels.
• Money may be spent to increase the amount of raw experience converted per long rest.
• Collector Treasure Sets, when sold, gain bonus sale value, and the bonus value also provides additional raw experience at the moment of sale if all items in the set were gained by the PC as treasure in adventures.
• Collector Treasure Sets may be kept, in which case they will increase the amount of raw experience that is converted to refined experience after each long rest.

This slows leveling to a degree but more importantly gives monetary treasure more significance and creates tension on how you want to spend it, or weather the gems and other art and jewelry items are better kept as a collection.