I can understand your problem as i faced similar issue when i started doing CR questions. Here is something you can do:
As i understood you are having problem while pre-thinking the answers in assumption questions.
You should practice first to breakdown the argument into conclusion(if any) and premises. Then think what is the necessary link that led to this conclusion based on the premises. They may be independent premises or may be dependent. In case of independent you have separate assumptions for each.
Assumption is like a missing piece of puzzle that will strengthen the conclusion and would provide new info to whatever has been stated in the argument.
Try to do this kinda stuff and you'll get better. Don't stress yourself for timing at the beginning. It will become second nature to you as you practice more and more.
Please let me know if you need more help.
PS:Please consider kudos if you find it helpful