I feel like I've seen this described before, but I'm not able to come up with the correct key words to find what I'm looking for.

We have a (v12) database for time entry and billing, and due to the huge number of records created, we've generally created "archive" files every 2 years or so. These are simply additional files, so that we don't have more than about 300k records in a single file - the system REALLY gets painfully slow around the two year mark.

I'm trying to find a way to run a report from my main database that will access the records in each of the archive files and pull them together in a single report - for example, report on all records with a certain activity for each month over the last X years, and hopefully display a graph showing trends based on those records.

Is a layout with multiple portals the way to go? Can I combine data into a single graph if done that way?

Do I need to import records into a master report file, run my reports and then delete the now-unneeded imported copies of the records?

(Is there a better way than the "every 2 years archive" system to keep my database from becoming painfully slow with too many records?)

I believe what you're looking for is what is called a virtual list.  Basically pulls information from several different tables and displays on one layout.  There are many techniques to build these types, but I prefer the one SeedCode published -  http://www.seedcode.com/virtual-list/