SEO's are generally the biggest contributor to slow files, in my opinion. One test would be to save out a separate pln and remove all operators/targets. If the file runs smoothly, thats the culprit.
The second thing to look at is the total polygons in the model. Depending on your platform/hardware, a high polycount can bog things down tremendously. I wrote another post, testing polygons on BIMx and found they had a bigger impact on ARCHICAD: https://wwabim.wordpress.com/2016/03/16/bimx-file-sizepolygon-tollerances/
Note that these tests were done on a brand new iMac, so if your hardware is even remotely antiquated, don't expect to get anywhere near 10 million polygons and function at all!
The third thing I look for on slow files is the library manager. Make sure you do not have excessive duplicate (or missing) objects. Make sure objects and images saved to the embedded library are as small as possible. A few 20mb png's attached to over a significant portion of the modeled surfaces can slow things down. Here are my thoughts on library management: https://www.linkedin.com/pulse/archicad-library-management-best-practices-patrick-may?trk=prof-post
Other suggestions that improve file speed/efficiency are the project map and layout book; clean them up and pare them down wherever possible. Files can grow a long stack of unused independent markers in the project map if left unchecked. This is very unlikely the cause of your 20 minute pauses, but these items are the list of checks that should be made first.