Hi Raffaele,
For complex models, the Hololens 2 does not have as much processing power as a laptop or desktop computer. There are a few tools in the Connect MR application to help with this. First try using the section box to reduce the amount of objects you are trying to view at one time. Secondly, you can also adjust the Performance/Detail setting - this allows you to sacrifice some performance (framerate) in order to view more detail in your model. This setting can be found in the radial menu under Navigation -> Settings.
For any further assistance please contact our support team: ftgsupport@trimble.com