There's your problem right there. You actually have, besides the OS SSD, a single 1 TB HDD that's been partitioned into three separate partitions. And the most used partition of that drive is also by far the absolute slowest partition of that drive! You see, no HDD can sustain high transfer rates throughout the entire disk to begin with. And when you partition that HDD like that, the maximum sequential transfer rate of the outermost partition is less than 90 MB/s (assuming that the maximum sustainable transfer rate of that given HDD is around 130 MB/s). And with that outermost partition so full, that transfer rate drops to less than 70 MB/s. And practically, that partition transfers no better than about 40 MB/s due to overhead from the controller.
Video editing demands high sequential transfer speeds to begin with. But a partitioned spinning HDD will only make an already slow drive even more sluggish.
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