It's never to late to do a little maintenance. Takes time (I went down for a couple of hours with 1.4M posts), but it is do-able.
But what really makes your hosting tick is the amount of traffic you get during the day, how much pressure you put on ram (mysql) and the cpu (alll other processes) and it doesn't matter if the 1 million posts are made by 200 or by 20.000 people..
Do you have any access to some statistics? I mean like the bandwidth you consume per month or a page where you can look at cpu load/memory use during you busiest moments of the day.