How should I investigate a slow computer which doesn't seem to use all available resources in the first place?

Question

From time to time my this computer slows down a lot. This is particularly noticeable for Blender, a 3D software, where rendering times of a couple of minutes are multiplied by 7 or more. Relevant elements:

• Using W7 (latest updates), Lenovo W510 (latest UEFI/drivers).
• Fan blades are clean, no dust on vents.
• CPU temperature 50°C, frequency around 930 MHz, 58% of the base 1.6 GHz.
• GPU temperature 44°C, core frequency around 500 MHz, 135 MHz when idle.
• There is plenty of RAM, 50% still available.

I've read: Computer is really slow. I made some measurements for two applications: Blender and FastStone Image Viewer. I started from a situation where the computer was slowww, and then just rebooted to restore the speed.

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Blender (a CPU consuming application)

Resource monitor before and after an image rendering task is started. Orange is Blender activity:

Left = Computer works normally -- Right = Computer is slow

When the system works normally, RealTemp reports the same value than Windows for CPU load (100%). When the system is slow, RealTemp reports only 29.6%. That's something I don't explain. Apart this, I don't see a significant difference between the two measures, at least a difference which can explain rendering time rising from 2 minutes up to 14.

Question 1: Does this difference between RealTemp and Resource monitor values indicate something that can help towards the solution?

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FastStone Image Viewer

Looking at activity while using FastStone Image Viewer is also quite surprising. While FSIV struggled to display the first image, I queued next/previous image commands to keep the application busy for a longer time:

Left = Computer works normally -- Right = Computer is slow

Again, while Windows reports 20% CPU utilization, RealTemp report 5%. But quite surprisingly, neither the CPU or the disk are actually overloaded while FSIV is unresponsive.

Question 2: How can I continue searching where is the bottleneck if CPU/Disk are not in the causes?

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Update

It has been days without the problem, but it reappeared while at the same time I experienced 2 device manager errors "unknown device". The two devices:

1:

2:

I uninstalled the two devices, then scanned for device changes. It came back with a single error, with the same string: "pci\ven_8086 dev_3b64 subsys_215f17aa rev_06" for the device ID, but now the message is more clear:

(translates as Simplified communication PCI controller)

It seems related to Intel ME. I actually unistalled IME shortly before this problem occurred as I assumed IME was used only to support Intel AMT, a remote management solution used in large companies (found on vPro platforms) which I don't want.

• As a side comment I think letting someone manage a system remotely including when it is "powered off" is nothing but a huge security risk, not to say a plain view backdoor.

I'm reinstalling IME to see if this solves the problem in the long term. Will update this question later. Any further confirmation/objection between IME not installed and the symptoms I described earlier are welcome.

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Sorry for the snapshots showing legends in French, this is hopefully easy to translate in English.