Several suggestions came to mind when I saw your post and took a look at your website.
The first was that you could look at speeding your iMac up by swapping the mechanical hard drive for a solid state drive (SSD). This may be a little involved as you'd need to remove the screen to access the existing drive but doing so should provide you with a boost to performance not only on startup but also in most general jobs.
In regard to pulling in and displaying your Discogs data I can think of three possible solutions that would offload the work you are doing in Freeway that is currently slowing you down;
1. Create an Action that pulls in the CSV data and spits out a table (or tables) exactly like your current pages. To update the site you'd simply choose a new CSV file and republish/upload the site.
2. Upload the CSV data file to your web server and do the same sort of table building using PHP when the web page is called. To update the site you'd need to FTP a new CSV file into the server.
3. Use Discogs own API to use PHP to pull the data directly from your Discogs account. This way there is no CSV file to down load and use as the data on your site will always be in sync with the Discogs listings. This is the trickiest to code up but one in place the listings would be identical over both sites.
By outsourcing the table building to an Action or PHP (on the server) you should be able to free Freeway up from the heavy lifting of creating and management of that data.