IOE is all the things you didn't learn about that hit you all at once and have time limits. You're fumbling with paperwork and things like that. After a couple pairings you won't even need a calculator anymore to do weights.

IOE is just overall confidence. Once you aren't wondering if you did something right you can focus on your job and you'll find you have a lot of free time once you get in the flow of things. Usually people spend some time punching around in the FMS and learning a little more about things the manuals don't really preach. I remember calling for a cruise check and the captain looked at me and asked if I was serious because he hadn't done one in almost two years lol.