Yeah it’ll be the big grandstand. A lot of people have performance issues at this track.
You can try a couple of tricks, firstly reset the graphics config using the UI > Settings option. You’ll need to set all the graphics options up again though, which is a pain. However it can help things if the game starts running slowly. You can also try setting it to the lowest graphics preset and then putting it back where it was before, this sometimes works too.
You can also try killing the iRacing service in Task Manager and then going into c:\program files\iRacing and running Start_iRacingService.bat to start it again. This sometimes magically fixes performance issues.
A good tweak if you’ve not done it is to set the Dynamic LOD FPS in the Graphics settings to your target FPS, that makes the game automatically reduce detail to keep the framerate stable.