Outbound chat CSV filtering takes too long

We’re trying to use the CSV filtering feature in the outbound chat to match phone numbers with our audience. But the process (tagging) takes way too long, 1+ hour for just 1 user, and still shows 0 people in the tags list. On the other hand, using the name as the matching attribute took only ~3min for the same user. I don’t think this is a format issue because the CSV file was created in notepad not Excel (which uses scientific notations). The Phone attribute is a text.