I haven't noticed a slowdown after using Dex. I use it all the time. No issues at all. Maybe there is something still running in the background you don't know about.
I had issues with my S10+ when I puttered around with DeX. I would have to reboot my phone to get it back to normal.
My daughter, on the other hand, hasn't complained about DeX on her S10e, and she has the 8GB/256GB model. Of course, she probably has used it twice for homework, so nothing too taxing.
Q: how are you using it? ie, what is it connected to such as a dock or hub, just USBC to HDMI, how are you getting power?
I wonder what shows in Device Care for memory used.
I read that support for DeX was coming to offer Note 10 level of support, so we're likely not there yet.
DeX definitely worked better on prior Note models than S8+ or 9, had plus and Note models for each. Office apps were spotty and unrefined then. Fun but no replacement for a laptop like experience. With the Note 8 I bought the DeX dock and hound it's Ethernet to be very slow option.
I'd wait now that Samsung has begun rolling out an update to the S10 line to improve DeX.
If I dont exit dex mode and go back to mirror before disconnecting i do notice some lag. But as long as i exit dex properly before it's fine with no lag..
I use my Dex daily. No desktop computer at home. Just a wireless keyboard/mouse and my phone. Just bought a laptop. Can't wait until the update that allows us to use Dex like the Note 10/10+ where I can download the software, connect the phone to my laptop, then run SamsungDex and have a window that has access to my phone. That is going to rock.
And I do not experience sluggishness after using Dex and I keep my phone docked all night long so it's on the Dex dock for sometimes 10 hours or more.