Prepare the Syntopic mind map before you start. Make it a focus of finding only the information that you need to put on the mind map.
Everyone of my PhotoReading class we do a Syntopic mind map. Students only have 30 minutes for fulfil a specific objective on the mind map. Naturally if they wanted to write a paper on their findings they would spend a total of 2 hours built up in layers possibly over a couple of days.
Check that you are following the process in the chapter on Syntopic reading. Each step and stage of filling in the mind map is explained.
AlexK