There may be a number of background tools or startup services running, which take up CPU resource priority over DSM. Try closing the non-essential processes through Task manager and then use DSM.
In DSM 4.0 and newer, adjust the rendering quality to 4 or 5 and anti-aliasing to medium via File-> DesignSpark options-> Popular.
Also, go into File-> DesignSpark options -> Appearance and switch to 'SpaceClaim 2007 (fluent)' ribbon style. See if this improves the responsiveness in projects.
If you still see no substantial performance gain and you have quite a complex design file open, then go to your design structure tree, create new components and move some solids/surfaces into these sub-groups.
From Advanced options, you can also switch to a different renderer like OpenGL to see if it improves performance.
You can also export your current design as a new RSDOC file (use a different name) and resume work on this file. This can help fix file bloat issues which can also cause slowness.