The slow down is CPU over load and at this point, a known issue. If you do a Ctrl - C, you'll see the graphic. It will be the purple bar. This video gives a quick over view It is one of the reasons i've been starting to look at upgrading.
LGC is hosting the final race of the GPLaps Historic Road Racing Championship next week. It's an excellent series. You can join in the preQuali week this coming monday-friday, with the race on saturday (there will be a broadcast). Check the series rulebook for info and signup instructions: https://docs.google.com/document/d/1TN0nLNfk3brLQVAXJml5aAePXU0iPsE6lzliACATcmo/edit
