MacBook13,3 extremely slow suddenly

My old 2016 MacBook Pro is extremely slow all of the sudden, starting a program like finder can take several minutes, every click on a button will have the UI freeze for a few seconds, installing a new fresh macOS version took over 20 hours - it is basically unusable. What I tried so far: - Completely erased the SSD, partitions, encryption everything - Re-installed the newest Monterey version from a bootable usb drive (the installation from the recovery partition failed) - Resetting PRAM and NVRAM - Reset the SMC - Run the system diagnostics on boot up (no problems found) - Checked the CPU DIE temperature, it is at 40°C and the fans are running Is there anything else I could try, short of bringing the MacBook to the shop?