why stops you from trying, the guide for the PC  lists what is sold new, only, dell does not keep testing newer CPU as they come out.  that is your job, or go PC benchmarks dot com and read, see what others tired, or gee google it?

NO CASE size told.

the restrictions are clear (some are)

Generation CPU supported. (the chip set tells  you that) Q87 Express Chipset

wattage supported (fan /heatsink sizes too and CPU VRM LIMITS)   see below..

BIOS support (never clear, but upgrade it first)

and mobo bandwidths allowed, (not spec 'd)

the spec pdf (a crude guide) shows only this.

Intel® 4th generation Core™ i7/i5 Quad Core (84W for MT & SFF, 65W for USFF and 35W for Micro),
Core™ i3 Dual Core and Pentium® Dual Core (35W for Micro only)

so buy one and plug it in, if fails downgrade, it.

https://www.dell.com/learn/us/en/rc803738/shared-content~data-sheets~en/documents~csg-en-xx-all-optiplex-9020-spec-sheet.pdf

not one thing told that is slow,  and speed matters,

what APP runs

read the q87 here, it is clear what it supports

read about the C1 stepping bug, USB3,  for sure. learn to wiki chip sets  the matching CPU first.

then learn to see if one stepping of any chip does something you don't like, Stepping's are MASK revisions and some times updates. all called out in the wiki great pages, to read.

https://en.wikipedia.org/wiki/List_of_Intel_chipsets

you can see GEN5 is not good for Q87 at glance there.  i7-4xxx ONLY, and 84watts or less it seems (dell is mum) the i7-4790K is max they ever made. gen 4, desktops.

https://ark.intel.com/content/www/us/en/ark/products/80807/intel-core-i7-4790k-processor-8m-cache-up-to-4-40-ghz.html

Welcome to the Dell Community  @Cman20032 

There are many.

What CPU do you have installed???

Best regards,

U2

Optiplex 9020 Spec Sheet

Spec sheet is for all 4 sizes.

Want to speed up your PC?  If you're thinking about the CPU, compare the one you want with the one you want at game-debate.com.  You might find not much difference from quad core to quad core.

Try this for upgrading in order - SSD > RAM > CPU

I had some improvement in performance going from a dual core to a quad in my 755 MT.  64% according to game-debate..

More noticeable improvent came when I maxed the RAM to 8GB and installed an SSD.  Boot time became a few seconds.  Unexpectedly, my USB's quit lagging and speeded up (2.0 & 3.0).  I couldn't be happier with the PC's speed.  Opening programs is instantaneous.  Shutdown is also only a few seconds longer than startup.  I also used to have freeze problems with the USB's if multitasked.  I could go on.

SSD's are one of the best ways to breathe new life into either an older machine or a machine that's starting to age.

I am currently running a Dell Optiplex 9020 (purchased 'barebones' on eBay) with a core i7 4790 @ 3.6 GHz and 16GB DDR3 RAM. Whole rig cost about $300 and works very well. I used the Dell Windows 7 re-install disk, Windows activated and I was then able to upgrade to Win 10. All for $300. Someone else could go even further with aftermarket GPU.

here is the Haswell list. ( desktops, look) 

if the chip  set has bug, dump the 9020 for better box. it has no socket..... that USB3 bug, on C1 die step.

or better MOBO.

q87 (die versions)

SR137(C1)
SR173(C2)
SR19E(C2)

not sure what is on your mobo but dell knows,  C2 is prime. some mobo makers put V2 on the mobo stamps.

https://topics-cdn.dell.com/pdf/optiplex-9020-desktop_owners-manual_en-us.pdf

Top end is the 4790T because the power supply is very small.

https://www.amazon.com/dp/B01CL6FQ90/

Intel Core i7-4770T

https://www.amazon.com/dp/B00DVDSQ0O

A link to the System Manual can be found here:

• Manuals and Documents for the OptiPlex 9020
• Manuals and Documents for the OptiPlex XE2

This section provides links to information about locations of connectors and ports, pictures of your system, and will help you to get started with your system.

• Visual Guide to your OptiPlex 9020 Computer
• Visual Guide to your OptiPlex XE2 Computer

why are you such a pain? 

I have a Core i7 4th generation in mine.

Best CPU for the 9020:

MT: i7-4790K (overclocking not supported)

SFF: i7-4790

USFF: i7-4790S

Micro/MFF: i7-4785T