brighthaireddumb Is the new save lagging too, or just the old one? If the new save is affected, with no mods or custom content—please remove your Mods folder if you haven't already—then I'd like to see a dxdiag. Click Windows key-R, enter dxdiag in the run box, wait for the scan to finish, click "Save all information," and save the file to your desktop. From there, you can attach it to a reply using the paper clip (Attachment) icon included with the other formatting buttons.
If only the old save is affected, you can try moving your sims to a different lot, or playing with different sims, to find out where the lag is coming from. This would be a test; I'm not saying you need to abandon your sims or existing builds.