If you have been wondering how an Xbox Series X will affect your gameplay, I hope to be able to answer that here.  I will be comparing the Series X with an XB1X with an external SSD (Samsung 840).

Load Times

Compared to the X with external SSD I would say load times for the Series X are 50-75% faster.  I have 18 characters and switch a lot so this is where I notice it the most.  If you have just one toon, it may not be a big deal.

Frame Rates

Obviously the performance difference is what you would expect.  I am running on a 4k 60fps HDR TV.  It runs as smooth as it does at 1080p on the XB1X with SSD.  Where I really notice a big difference is the map.  The map on the XB1X is pretty sluggish and is especially painful if you are in Grana and need to look at something in Valencia.  After playing a while it gets worse and is very laggy.  On the Series X, it is almost too fast and is smooth as butter.  If you have a worker army, feeding them seems to slow down on the XB1X.  On the Series X it is super fast.

Memory Loss

Recently I have been doing a lot of horse training overnight.  I let it run overnight and in the morning it is so laggy that I need to totally exit BDX and restart.  So there is some memory leaking going on and gets worse over time with the XB1X.  I have yet to see this on the Series X.  It just runs smooth all the time.  Maybe because the hardware just brute forces it's way through or maybe it just handles loading/unloading textures etc way better.  Either way, i havent experienced this lagginess after running overnight.

Audio

I run ATMOS on the XB1X and when I turn my TV off and then back on, the audio sounds "stratchy" and very loud.  The only way to fix it is to restart the game.  I am running DTSX on the Series X and do not have this problem when I restart my TV/reciever.  

Noise and heat

The Series X seems to run much quieter and cooler than my XB1X

This is a quick and dirty look at some of the differences between my XB1X and Series X.  Let me know if you have questions or you want me to look at something specific.  All in all I love the Series X and feels like a good successor to the XB1X.