Question CPU utilization dropping from 100% to around 65% while rendering a video in Sony Vegas

One 16 GB DIMM, but this wasn't an issue 1.5 years ago
One 16GB DIMM WILL limit your performance. Video rendering can only happen as fast as data can be brought into the CPU and sent back to memory for eventual writing to disk. You only have 1/2 the memory bandwidth that CPU was designed to have.
I have no idea what has changed in 1.5 years for your PC. Your statement is like "I could run faster 10 years ago."
If you want the fastest system, do a clean Windows install. Then save your $$$ to buy a matched set of two 16GB DIMMs.
 
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Your yo-yo cpu usage pattern suggests thermal throttling.
Run HWmonitor.
If you see some cpu cores at 100c. in red, it indicates throttling and lower cpu performance.
This will persist until the temperature resolves and the cycle repeats.


What is the make/model of your case, and what is the fan arrangement?

What cpu cooler are you using?
If you are using the stock intel cooler, look to see if it may have come loose.
All 4 push pins should be through the motherboard and locked.
It may not be level. You need to push down on DIAGONAL pairs of pins at the same time to get the cooler level.

If you remount, you need to clean off the old paste and apply new.
 
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You also have ~40% usage on your GPU so if you use GPU encoding it could be normal for the CPU to slow down whenever the GPU gets to work on a new piece of data.

Heat issue would result in clocks going down and utilisation would stay at 100% all the time.
 
You also have ~40% usage on your GPU so if you use GPU encoding it could be normal for the CPU to slow down whenever the GPU gets to work on a new piece of data.

Heat issue would result in clocks going down and utilisation would stay at 100% all the time.
Yeah, the problem was with the GPU. But it's faster with the Nvidia Encoder than without.
Without the Nvidia Encoder, it's way slower, but the utilization doesn't jump between 100% and 65%.
 
Your yo-yo cpu usage pattern suggests thermal throttling.
Run HWmonitor.
If you see some cpu cores at 100c. in red, it indicates throttling and lower cpu performance.
This will persist until the temperature resolves and the cycle repeats.


What is the make/model of your case, and what is the fan arrangement?

What cpu cooler are you using?
If you are using the stock intel cooler, look to see if it may have come loose.
All 4 push pins should be through the motherboard and locked.
It may not be level. You need to push down on DIAGONAL pairs of pins at the same time to get the cooler level.

If you remount, you need to clean off the old paste and apply new.
The CPU's temp's are fine, below 70 °C.
 
@C1rm0ska
The Utilization data that the Task Manager graphs is not the same as CPU usage. On Intel CPUs that use a high percentage of turbo boost like the 11400F uses, this will create some exaggerated utilization values compared to actual CPU usage. The Task Manager Details tab reports CPU usage.

Utilization is calculated using the base multiplier which on the 11400F is only 26. When the CPU is using the 42 multiplier, the CPU usage is inflated by 61.5%. (42/26 = 1.615)

Compare the ThrottleStop C0% data to the nonsense utilization data that the Task Manager reports. The time a CPU is spending in the C0 state actively working on a task is the best indication of what a CPU is doing. The Windows graphs look nice but the utilization data that is being graphed is junk.

The C0% is probably somewhere around 50% when running Sony Vegas. That means it is only actually using 6 of the 12 available threads on your CPU.
 
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To see individual thread use, right click on the cpu graph and select to display logical processors.
That is not going to show individual threads, it's going to show what usage that one, logical or not, core is doing which is work from potentially thousands of threads.

To see individual threads you need process hacker or process explorer double click on a process and go to the threads tab that will show you how much CPU a thread is using, independently from what core it runs on, unless you force it to run with affinity on a certain core.
Utilization is calculated using the base multiplier which on the 11400F is only 26. When the CPU is using the 42 multiplier, the CPU usage is inflated by 61.5%. (42/26 = 1.615)

Compare the ThrottleStop C0% data to the nonsense utilization data that the Task Manager reports. The time a CPU is spending in the C0 state actively working on a task is the best indication of what a CPU is doing. The Windows graphs look nice but the utilization data that is being graphed is junk.
Is that much of a difference?
If the CPU is running 100% usage but at half the speed it's still only half used because it could be running 100% at 100% speed.
 
If Sony Vegas is similar in performance terms to Adobe Premier Pro, I believe you could benefit from more RAM as Titan mentioned.

Running two 16GB DIMMs in dual-channel mode would give you double the memory bandwidth of your current single-channel setup. I'd expect at least a 30% improvement from switching to dual-channel and 32GB should give more headroom.

Puget Systems recommend 32GB RAM for 1080p, 64GB for 4K video and 128GB for 6K/8K+ in Premier Pro. The latest version of Sony Vegas might have similar memory requirements.

https://www.pugetsystems.com/soluti...be-premiere-pro/hardware-recommendations/#ram

Depending on what video editing apps I'm using, I've observed some programs using far more GPU than CPU processing power, especially when OpenCL is used. At other times, my CPU is running flat out whilst the GPU sits idle.