Without a lot more detail, we can't help. My Windows 7 machine with 24 GB of RAM does not do as you describe, so something is different between our systems.
What is in the project you've opened?
Does it do it on every project or just that one?
What's the exact version number of your AE? I know you said that it's CC 2015, but is it 13.5.1 or 13.6.0 or 13.6.1 or...?
What are your memory settings in the preferences? (You could take a screenshot if you want.)
Does that same project behave in AE CC 2014?