Discussed in T407782: Donor Portal/Civi visibility for Donor Relations
"A more efficient approach from a data perspective would be to record the last time the donor logged in, versus storing an activity for each specific time they logged in. We could display this around the Donor Portal link already on the CiviCRM contact summary page. This should help donor relations quickly identify the donor's activity and serve the same purpose while reducing the amount of activity data needed to be stored in the database."