Thanks for attaching the screenshot. VisualGDB should never color the regular Output window (hence we wanted to double-check you are not still accidentally using Advanced output).
We have rechecked the coloring behavior with the latest VS2019 16.6.2 and could not get the coloring shown on your screenshot. Please double-check that you are not using additional extensions that could be coloring the output. You can disable various extensions via the Extensions->Manage Extensions command.
If it doesn’t help, please try disabling the VisualGDB extension via that command and restarting VS. If the project is based on GNU Make, you will still be able to build it (although VisualGDB won’t be loaded into the VS and should not interfere with the IDE responsiveness). If this still results in a slow behavior, please try creating a regular Win32 console project and check if building it slows down VS as well.