Hi bertrofski64, and welcome to the forum!
Thank you for your report
we are currently investigating issues with the game being slow and laggy. It would help us greatly with this investigation if you could give us a little more detail about which areas of the game are slow for you, and also some system details, especially your browser, Operating System, CPU (Central Processing Unit) and GPU (Graphics Processing Unit), thank you.
Please also let us know whether your Hardware Acceleration setting is On or Off, and whether switching to the opposite setting improves things for you (you can find this setting in your Options menu in game accessed via the 'gear wheel' icon at the bottom right of your city screen).