Definitely vote for the idea. It'd be a good enhancement.
In the meantime, here is a potential solution:
1. Create a custom score field, e.g. "Days before Known".
2. Create a trigger campaign that triggers on Visits Web Page AND checks if 1) Lead is Anonymous AND 2) "Days before Known" is empty. Add Flow step to set "Days before Known" field = 0.
2. Create a daily batch campaign that checks if Lead is Anonymous. Add Flow step to increase the "Days before Known" field +1.
3. Add "Days before Known" field to Lead Analysis as a metric.
4. You can now create a Lead Analysis report to get the average "Days before Known" grouped by Lead Created Year/Qtr/Month/Week in Revenue Explorer.
Initially, for existing anonymous leads, "Days before Know" will be smaller than what it should be, but going forward, new anonymous leads will be correct.
Let me know if you have any questions.
TJ