Turbo Boost is specifically power limited by INTEL.  Its marketing.

You NEVER get full speed when all the cores are running.

When turbo boost is running it ONLY boosts 1 core.

Intel® Turbo Boost  depends upon a number of factors including, but not limited to, the following:

Type of workload
Number of active cores
Estimated current consumption
Estimated power consumption
Processor temperature

Intel® Turbo Boost allows the processor to operate ONE CORE at a power level for short durations

(A few seconds OR LESS)

You can monitor this with the Gadget

https://software.intel.com/en-us/articles/intel-power-gadget-20

Has nothing to do with new features. The SOCKET keeps changing because INTEL wants to sell more and more memory and cpus and chipsets.

https://i.dell.com/sites/csdocuments/Shared-Content_data-Sheets_Documents/en/Optiplex-390-Technical-Guidebook-101311.pdf

Intel® Core™ i7-3770S

390 does have a bios update with new microcode.

You cant go directly to the latest from A00 however

https://downloads.dell.com/FOLDER01799737M/1/O390-A11.exe

The 390 is a crippled system with ATA only.  AHCI is not an option. There is a 2018 issued bios patch for Spectre / Meltdown.

The 2011 Optiplex 390 was only validated with the following i3/i5 CPUs =

CPU socket type: LGA 1155

MHF7R i5-2400 Sandy Bridge, 3.10GHz, 6MB, 95w, 4C, D2
950R3 i3-2130 Sandy Bridge, 3.40GHz, 3MB, 65w, 2C, Q0
972H5 i3-2120 Sandy Bridge, 3.30GHz, 3MB, 65w, Q0
5MH9T i3-2100 Sandy Bridge, 3.10GHz, 3MB, 65w Q0

Dell does not support any cpu other than the one that came with the system.

The 390 has an Intel H61 Express chipset which is different from the 790 and 990.

Whether or not the I7's work is up for debate.

If you DO NOT HAVE A TOWER any unit other than the "S" models will likely break the voltage regulator on the motherboard.  The smaller units do not have sufficient power.

So the i5-3340S - CM8063701387400 / BX80637I53340S  might work as would the

i7-2600 - CM8062300834302 / CM8062300834303 / BX80623I72600 / BXC80623I72600
i7-2600K - CM8062300833906 / CM8062300833908 / BX80623I72600K / BXC80623I72600K
i7-3770 - CM8063701211600 / BX80637I73770 / BXC80637I73770
i7-3770K - CM8063701211700 / CM8063701211701 / BX80637I73770K / BXC80637I73770K
i7-3770S - CM8063701211900 / CM8063701211901 / BX80637I73770S

This must be done via XP or 7 WITH NO ANTIVIRUS SOFTWARE INSTALLED.

Disabling does not work it must be removed and the system rebooted.  Then RUN BIOS UPDATE as Administrator.

Failing to heed this warning can lead to Bricked motherboard.

http://downloads.dell.com/published/pages/optiplex-390.html

You cannot jump from A00 to A14 in bios so which depends on how old it is.

Description	Version	Download
Dell OptiPlex 390 System BIOS This package contains the BIOS update for Dell OptiPlex 390 that runs Windows and DOS operating systems. BIOS is a firmware that is embedded on a small memory chip on the computer's system board. It controls the keyboard, monitor, disk drives and other devices. This package contains update for CPU microcode to address Intel Security Advisory INTEL-SA-00115 (CVE-2018-3639 and CVE-2018-3640). The package also updates to the latest CPU microcode to address CVE-2017-5715.More details	A14
Dell OptiPlex 390 System BIOS This package provides the BIOS Update on the Dell OptiPlex 390 running the following Operating system: Windows and DOS.More details	A11
Dell Optiplex 390 System BIOS This package provides the BIOS Update on the Dell OptiPlex 390 running the following Operating system: Windows and DOS.More details	A10
Optiplex 390 System Bios OptiPlex 390 System BIOSMore details	A09
DELL,DSK PROG,BIOS,FWH,390,A07,A07 OptiPlex 390 System BIOSMore details
DELL,DSK PROG,BIOS,FWH,390,,A05 OptiPlex 390 System BIOSMore details
DELL,DSK PROG,BIOS,FWH,390,A03,A03 -More details	A03
Dell OptiPlex 390 System BIOS, A02 This package contains the Dell system BIOS update for OptiPlex 390 system. BIOS is a firmware that is embedded on a small memory chip on the computer's system board. It controls the keyboard, monitor, disk drives and other devices. This update includes the latest CPU microcode, Intel Management Engine Firmware and fixes the issue where the hard drive doesn’t get detected.More details	A02
Dell OptiPlex 390 System BIOS, A01 OptiPlex 390 system BIOSMore details	A01
Dell OptiPlex 390 System BIOS, A00 OptiPlex 390 system BIOSMore details	A00

16 gigs is not an option if you only have 2 Ram Slots.

The 990 has 4 ram slots and allows 16 gigs.

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-990-desktop

SFF is 8 Gigs Max end of story

8 Gig Dimms are NEVER an option.

Saying you can get to 16 gigs without restriction is misleading at best and a pre meditated lie at worst.

The ram must be LOW Density INTEL compatible and not be ECC or Registered or Buffered or Quad Rank etc.