Latitude 7420 has a CPU frequency limit of 1.5 GHz (presumably due to the BIOS firmware (1.47/48.0)).

System  info (From BIOS 1.48.0):

Minimum Clock Speed: 0.40GHz

Maximum Clock Speed: 4.40GHz

Current Clock Speed: 0.40GHz

Processor Type: 11th Gen Intel(R) Core(TM) i5-1145G7 @ 2.60Ghz

Memory Speed: 4267MHz

Battery:

Health: Excellent

AC Adapter: 65W

Background and current problem:

I received this laptop with Windows 10 and BIOS version 1.28.0. It had the issue described here(https://www.dell.com/support/kbdoc/en-us/000255870/latitude-7420-or-7520-cpu-frequency-drops-to-lower-than-1-ghz-when-running-on-battery) and some problems with power plan in OS. After upgrading to Windows 11(including BIOS capsule update), installing all the necessary drivers, and configuring the system, the original issue was resolved, but a new one appeared: the processor frequency wouldn't rise above 1.5 GHz. Also the BIOS  displays that the current clock speed is 0.40 GHz.

What I tried to solve the problem(and it didn't help):
1. Reinstalling drivers, disabling Intel DPTF, disabling Intel PPM (sometimes helps when CPU stuck on 0.40GHz)
2. Using a different OS (Linux)
3. Changing BIOS configuration: I tried various combinations of disabling/enabling SeedStep, SeedShift, C-States; and also changing the Thermal Management  setting (UltraPeformance and etc).
4. Testing the system using F12 boot menu: Both tests (Quick and Advanced) were successful.
5. Resetting (to Factory) the BIOS, reinstalling the BIOS (using F12), and rolling back to 1.47.0 (other versions don't available for rolling back) .
6. De-energizing the motherboard(disconnect the battery)

Additional information:
1. This is not thermal throttling, as during stress tests  with each core running at 1.5 GHz, the temperature does not rise above 49 degrees Celsius in UltraPerformance mode (and the fan does not run at maximum speed).
2. Specialized programs in both Linux(s-tui) and Windows indicate that the maximum frequency at which the processor can operate is 1.5 GHz.

Conclusion:

It seems to me that the problem is in BIOS or, at worst, in hardware, but not in OS.

As far as I know, the 1.5 GHz frequency limit applies to the "Quite" and "Cool" thermal management modes. Therefore, the problem may be due to incorrect operation of the "UltraPerformace" mode.