It might not be but try to clear up your media and cache in the preferences.
Also check what render is being used in the composition settings.
Some projects are going to be laggy no matter how powerful your hardware is. Complex things are slow. If you're new to AE, sometimes it can seem like things are wrong, but that may be normal. Templates are often very complex. What template was it?
I have clean the media and cache ~ but still.......@_@
because the author also found out the CC version have a slow loading progress on their template,
that's why they suggest me to use adobe CS6 version.
the Main reason I bought the template because the character they created is so cute suitable for me
to make a video explainer myself to promote my corporate gifts website, so I need some expert here
to give me a guide to solve this issue T_T
Did you try CS6? It's included in your CC subscription. So is the original CC release as well as CC 2014, CC 2015, CC 2015.3, and CC 2017 - and they can all be installed alongside each other without issue. See this page for how to download previous versions (they're not where it looks like they would be).
Anyway, if CS6 works, I'd try using CC 2014 too.
Try and do what Szalam mention, also you can work with the project on a lower res playback like quarter.
Anyhow some projects are just too heavy, specially if you have a lot of complex stuff going on. O saw the template and it seems to have a lot of layers, characters etc.
I have a high end desktop and I still have some laggy projects.
Did you ever fix your lag on loading files? Let us know if changing versions of After Effects helped at all.