High CPU usage on boot menu in triple-boot setup

I was dual-booting Xubuntu and Windows 10 on my T450s. Both on a single HDD on different partitions. The boot partition is on this HDD too.
I added a separate SDD and decided to install NixOS on it.
Here is lsblk after installing (sdb is SSD):

sda      8:0    0 465.8G  0 disk
├─sda1   8:1    0   500M  0 part
├─sda2   8:2    0   100M  0 part /boot/efi
├─sda3   8:3    0    20M  0 part
├─sda4   8:4    0   ...G  0 part
├─sda5   8:5    0   ..G  0 part
└─sda6   8:6    0   ...G  0 part /
sdb      8:16   0 238.5G  0 disk
└─sdb1   8:17   0 238.5G  0 part

1- Upon power on, I see a spike on my CPU usage. It becomes so bad that fan starts up (The air coming out of vents is actually hot so it’s not some firmware routine). This was not happening with GRUB (Xubuntu+Windows 10). But this triple setup is using sd-boot. What “process” is possibly keeping CPU busy on boot?

2- The Windows option does boot but is problematic. No desktop icons is rendered and no app can be run. Even the task manager can’t be executed. The thing is, this high CPU usage persists in Windows session. It does not happen on Nix or Xubuntu. So could this problem be related to my boot problem?