Slow Finder - Caused by external disks?
You can try unchecking "Put the hard disk(s) to sleep when possible" on System Preferences > Energy Saver (source).

There's also an app that says it will give more detailed control called (appropriately) "Keep Drive Spinning". 
Here is a link to the developer site, the app sends you to macupdate for the download –be careful with bundled software on the "install now" link. I have not tried it myself.