Question Macbook Pro 15" (Mid-2012) is dead slow

Sep 4, 2019
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Hello All,

My Macbook was running well until a month ago. But now it's dead slow. Even took hours to install Mac OS High Sierra from scratch.
I ran First Aid on SSD, it reports no problem. I ran default tests (4 Passes) of Memtest86 which took almost 9 hours, even that didn't report any errors. (Though the test was on Single CPU as Parallel CPU Test was not available due to UEFI Firmware limitations).

What could be the problem now? It's working but takes hours to process a simple task. Please help.