You guys forgot probably the most common issue. Underperforming computer. This issue happens in a beginning city and slows down over time as more power is required to keep the burgeoning city processing.
If the OP goes to the options menu and graphics settings and set everything to low/min/off
Screen Resolution: 1024 x 768 for std monitor. 1280 x 720 for wide screen.
Lighting: Minimum
Textures: Low
Shadows: Off
Geometry: Low
Animation Detail: Low
Anti-Aliasing: [Unchecked]
Framerate Cap: Off
V-Sync: [unchecked]
This should remove all lag
If you can post what your computer's name and model number are, we can help you much better instead of guessing.? i.e. Dell Inpiron 518