There is mainly two reasons why you asking yourself why my computer is slow. Hard drive and virus. To diagnose the hard drive you need to get a diagnostic software. To confirm that this is a virus infection you need an antivirus software. That antivirus that is on the computer probably do not work anymore because very often viruses disabling antivirus software.
Dust is the enemy number one on computer systems. Please double check everything inside computer is clean. Especially to the CPU heatsink.is it clean and is the fan working. There is a reason why overheating computer is slow. The CPU and motherboard are designed with capability to slow down CPU’s speed if the temperature reaches critical level. If the computer is overheating it puts heavy load on the power supply. The computer that works fast and is not overheating is always clean as shown in image below.
Not enough memory or memory has a defect. The memory defects are rare but it happens occasionally. Especially on older computers and on computers with no brand names. Also it is common for not famous RAM module manufacturers and for modules that look fragile from outside and are very light.
Sometimes the PSU is the reason why computer is slow. Usually this is a capacitor problem. And what it does defective PSU is that it supplies motherboard and all the other components with a power that is not stabilized, with spikes or fluctuations.
The power spike is the number one enemy for the hard drive. If you have replaced the hard drive and the new one also failed the power supply is to be blamed. This kind of power supply fault will not show too much signs. Just with the oscilloscope we can identify such fault.
Too much software on the system like bloat ware that came from the factory and all the malware or adware that got installed during the exploitation. All the registry optimizers, speed optimizers, bogus antivirus software (yes there is a virus that advertised as an antivirus) etc. such software uses resources and definitely makes impact on system performance.
Motherboard defects. This is rare fault it happens especially if the motherboard is older than three years. It can be widely known capacitor problem, soldering problems in the result MOSFET’s are desoldering themselves slowly, other design errors that let motherboard components to overheat like always hot North Bridges and South Bridges etc. here is an input/output controller on Acer laptop that is very close to the hard drive.
Because it was so close to the hard drive the hard drive was constantly overheating and in the result the disk was damaged. This is I would say a design flaw.
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