We have the same issue. I think that it imports a lot of DXFs and DWGs in a way that creates a large number of short lines or curves. If I have time when importing footprints i delete all unnessesary lines and sometimes even redraw sections. I know that probably doesn't help you much but I do experience the same thing and it seems to be in all versions of the software I have used.
Something to understand about SWE....
First, you can actually DELETE the DWG files for each page and when you click to open said page, it will be regenerated (except for some dumb graphics not "registered" in the system...
If you have unregistered symbols in the system, it will scan everything you select and search against the database. There's no other way to know if it is working on an actual SWE item or not.
Ways to help/fix these 'related' issues:
1) Make sure the imported DWG/DXF items are already in a BLOCK. This helps.
2) Convert it to a passive library symbol.
When I worked with the Trace team (makers of this software) I petitioned a new symbol classification "Passive Symbol" to help with this and give you a way to manage symbols in the library that had no need referenced/managed attributes. In fact, the only thing you can do when editing such a symbol is change it's insertion point or work with "personal" attributes you have place in it (for custom text, etc). They also work VERY WELL for the side and bottom view of your panels. You only need place the views you need instead of having all views or no views. Remember, they do not have any system attributes.
If you'd like a good example of this symbol type, do a symbol search for BUMP. This will give you all the little symbols that help show there is no connection point between crossing wires (remember to disable wipeout frames in in drawing parameters for any page that uses the bumps. .
Hope this helps.
Also, 2018 will have 2 new options to help with performce.. Currently the software uses the standard WinGDI library. There are 2 new graphics options for the software drawings... These will be OpenGL and DirectX. 2018 has many new features and had a big push on performance from the developers.