SIMCITY 2013 FIX
There is a lack of SimCity 2013 tips and fixes out there on the web these days. It seems most have decided not to play it anymore and fix it. I don't blame them because it's a very buggy game. In case there is one person out there playing this other than me and has an issue with a city suddenly running slowly I have found a fix. Clear the cache files out for the two locations. Just delete the two files.
C:\Program Files (x86)\Origin Games\SimCity\SimCityUserData\Cache.package and C:\Program Files (x86)\Origin Games\SimCity\SimCityUserData\GraphicsCache.package
That will fix the issue. It did for me. You might want to go and delete them again every play or two to keep your cities running smoothly being they don't update the game anymore. It's a shame they abandoned it.