If those were the answers the Genius at the Apple Store gave you, they are far from being a genius it seems since the answers were nonsensical. Very akin to a shady used car salesman trying to take advantage of a novice by selling them something they don't need.
Anyway, click on the Apple icon on the top left of your screen and choose About this Mac. A dialog will appear showing the basic specs of your machine, if you can take a screenshot and paste it here, do so. If not, just enter the information you see on the first tab. Then click on the Storage tab and tell us what is says for your Macintosh HD, it should say "XXX available of XXX GB".
Next, launch Activity Monitor, and select the CPU tab. Now click on the % CPU header to sort the apps on the ones using the most CPU. Take note of the System, User, Idle values below. If, as you stated earlier, the Idle is at or above 90%, then your system is largely sitting waiting to do work. Now jump over to the Memory tab and the Physical memory is how much total memory you have, Memory Used is what's been used up from your applications. There should be a breakdown of App, Wired and Compressed associated with that. Wired memory is basically memory that locked down and cannot be reused. App memory is memory in flux that can move between applications and Compressed is what was recently used but not currently in use but could be used again, so it's compressed ad saved away.
Swap is amount of time the system has run out of memory and has needed to make space by copying stuff out of memory to the HD. The lower the number, the better. You are doing well with 270MB of that.
If the Memory Pressure is less than half the chart, you are doing good there as well.
Now, tell us if the slowdown exists immediately after a restart? Also, besides Safari and Excel what other apps are normally running?