I don't think I've ever seen 70+ extensions per card, but I would be surprised that it would have any real impact on client performance. Check out the "CardExtensions" table in the database. It is just a lookup table.
I imagine PDM just looks for the extension, finds the card ID and ProjectID and displays the card defined in the "Cards" table.
I didn't even think to check for Card tables It has a CardID which goes back to the Cards table which has a ProjectID. I'm sure there are some SPs that process all of this and could take a performance hit.
select extension, ce.cardid, cardname,path
from cardextensions ce
inner join cards c on c.cardid=ce.cardid
inner join projects p on p.projectid=c.projectid
order by cardname
Well, not exactly...that query wouldn't work if a sub folder was selected....but yeah that is along the lines of what I was thinking too. In short, the number of extensions referencing a card wouldn't have much on the performance.