Sometimes when importing a large DWG or DXF to a 2D Sketch, the amount of geometry can cause the graphics performance to decrease. This is partly due to the large number of profile condition locators and constraint symbols on the geometry. To improve performance, right-click the Sketch grid and go to Show. Turn off profile condition locators and constraints and click OK. This will remove the symbols and improve performance. You can turn them back on later if needed when working with a smaller Sketch. Another alternative is to use the "Import Reference 2D" command that can be used to import large data sets and can be referenced by other elements in the 3D to construct geometry in relation to the imported data.