Optiplex 390 slow with Win 10

Also, I have an i3-2120 CPU @3.3Ghz

Yes it is 2nd gen Sandy bridge cpu and chipset, significantly slower than 8th gen cpu, but faster than even older core 2 duo, which still supports Win 10.

if you have an i5, upgrade to top cpu of i7 may help a tiny bit.

if you have hdd, upgrade to sata ssd will speed up boot/restart significantly.

re: the machine “technically” doesn't support Win10.
chronologically machine was originally marketed for Windows 7.  Its hardware supports Win 10 64 bit natively.

Will the SSD upgrade improve general performance or just startup/restart speed? 

@redxps630 Thanks for your help. Glad to know the i3 is the issue. Where does one fine a 2nd gen i5/7 these days? 

Hi,I see you’re looking for technical assistance. If you require our help, you could initiate a private/direct message with us, and we’d be glad to assist you.

SSD would not help single threaded processor performance.  8th gen cpu has a much higher cpu benchmark score than 2nd gen.  2nd gen is not bad for email and surfing internet.  In my experience the slowest part of Windows operation is when it does automatic update or some other background tasks that you don’t even know, which can slow down front end task. 
check task manager/performance for cpu/ram usage %. If they are close to 100% during slow performance, you may consider upgrade them.

on ebay if you see a good deal.

i5-2500.  

i5-2400

i7-2600

if your stock heatsink cold plate is aluminum, you will probably need to get a copper cold plate Dell OEM heatsink because i5/7 TDP is 95W higher than 65W of i3.

One of the differences between the i3-2120 and the i7-2600 is 3.3Ghz and 3.4Ghz with 3.8Ghz turbo respectively.  The main difference between these 2 is the quad can handle loads better.

I'd still try using an SSD first.  It's the easiest bet.  It does speed things up, not just boot times.  Whether you try SSD first or second, you'll get the most notable difference out of the SSD.

I went from dual to quad in my previous 755.  Game-debate showed me 64% improvement between the CPU's.  I didn't notice much difference until installing and SSD.  They really do breathe new life into older machines.

If your 39O once worked quickly with Win10 and later slowed down, it sounds like it's time for a reload anyway.

@redxps630 I have an i5-2600k and larger heatsink ordered and on the way. I will let you know the gains once installed. It was all of like $45 on eBay and way cheaper than a new machine. 

Spend another $30 or so for an SSD Trust me, you will not regret it!

I'd like to add that if a PC is working herky-jerky, going from a dual to a quad shoukd definitely help.  SSD will speed it up even more.  Quad and SSD will make a great combo.

The CPU finally showed up today and I put it in and its working and showing improvement. I did a Performance Monitor test and am getting failure System Service Checks failures. Abnormally Terminated Services Check failed. What would cause this? Also, it shows missing events in event log and a service is reported as having an unexpected error code. 

I did not run this with the i3 but maybe should have to see if there is a difference. 

So it's working fine except you get errors if running tests.  Let's try resetting CMOS.  Remove the battery for 10 min. and then reinsert.  If the battery is 5 or more years old, it's time to replace it.