By the way, the file that you have opened super fast. Nice PC!
If you experience any slowdown due to the increase of the mesh density, you can go to the settings of the display mode and increase this one to 100%. That will ignore the shadows, but they will be still active. The framerate should be faster that way, but you will no longer see the gentle shadows that contribute to the perceived depth of the 3d model.
Disabling the shadows entirely also improves the framerate, but this comes at the cost of a totally different appearance of the rendered material and a very flat image, so I recommend to leave it enabled.
If you change “Density” from 1 to 0, that will reduce the mesh density (typically 4 times reduction of the polygon count, but depends on the overall shape).