Question Slow POST problem on ThinkPad E15 Gen 3 ?

[iammidnight233]

For a couple of months my Lenovo ThinkPad E15 Gen 3 AMD (with Ryzen 5800U & soldered 8G & 1x so-dimm ddr4, AMI UEFI firmware) seems to work really fine like before, except for one fact that it takes over 30 seconds to launch my bootloader (systemd-boot menu of Arch Linux, if that matters). The strange thing is that it's NOT the case where the Lenovo logo appears a long time after pressing the power button. Instead it IS the case that the logo itself instantly shows up after powering on, and (after prompting for boot interruption) it just stays there for a LONG time before bootloader kicks in.
Other details:

1. If I interrupt the boot process immediately after powering on (press Enter), the interrupt menu pops up immediately too, if I select a boot device, "the daze" happens afterwards.
2. But if I wait for at least 2 seconds, "the daze" happens before the interrupt menu pops up.
3. `systemd-analyze` shows `Startup finished in 30.838s (firmware) + 3.064s (loader) + 1.251s (kernel) + 4.562s (userspace) = 39.716s graphical.target reached after 4.562s in userspace.`

Hardware I modified:

1. Upgraded the RAM (from 8G soldered + 8G so-dimm to 8G soldered + 32G so-dimm, DDR4-2400)
2. Once I installed another NVMe to the spare slot, but soon for reasons I removed it. (It mostly happens before the problem appears.)

Things I tried:

1. Updating to the December firmware (just did that, nothing changes)
2. Resetting the firmware settings (no effect)
3. Tweaking the boot order (set NVMe or Linux to top, no changes) (problem persists even when I boot with a thumbdrive)
4. Installing other OSes such as Windows (no effect)
5. Disabling every security & integrity related settings in FW (no effect)
6. Disabling AMD-V and memory protection (no effect)