[ OK ] DietPi-Benchmark | Running benchmark suite tests, please wait...
[ OK ] DietPi-Benchmark | Running DietPi-CPU Benchmark with max value of 1000000 over 8 cores, please wait...
0 125000
125000 250000
250000 375000
375000 500000
500000 625000
625000 750000
750000 875000
875000 1000000
2778
Edit: seems to be solved by changing power supply. Initially it was 5V2A noname source successfully used for other projects, now it is Ugreen charger with 2.4A output.
@AndrewZ
Your external SSD does not have a dedicated power supply? I highly recommend, especially with root fs on it, since a USB port is usually not made to power more than a USB flash drive, especially on SBCs.
Generally you can run dmesg to check for kernel error messages related to low voltage.