As discussed in this post People rarely use the "Mark forums read" link on index page or the link of a forums page so these tables can grow quite large.

In the mentioned post it provides some SQL to set all topics 1 year or later than current date to read for everyone. 

I would suggest that an improvement to phpBB would be to add this functionality to the ACP that that the board admin(s) could establish their own limit on how far back to mark all topic read for everybody.