Libraries taking time to download in CF 2018 update 5

You're asserting that it's CF making js and image file downloads take longer. It's possible (indeed likely) that CF is not involved in serving those at all.

• Are you using a web server in front of CF, like IIS or Apache? If so, THAT is what's serving those files. Or are you seeing this with the CF built-in web server (such as on port 8500)?
• Assuming you are using a virtual directory to point to the scripts, is the actual directory on the same server as the web server (and CF) itself? Or is it some sort of network share? What about the images?
• Another question woud be: when you say the download is slow, are you viewing that speed in the dev tools of your browser, in its network tab?
• Finally, besides upgrading from CF10 to CF2018, did you also change servers, such that CF10 was on a different machine than CF2018? In that case, what else changed? Operating system? Web server version? Anti-virus or firewall? Any of those could have an influence.

And you mention being on update 5: just note that there is now an update 6 since a week ago, and most people should update to that to resole problems they had due to update 5. Note as well that you should update the web server connector as well, though again I doubt these would help with the problem you are reporting.

Let us know what you find.