Is encryption speed suppose to get lower and slower the longer it's running?
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I'm trying to encrypt my entire 5TB external HDD using a full encryption using AES-256 and SHA-512. I left it alone and came back saying it would take 9 days to fully encrypt with speed down to 11-14 MiB/s. The speeds starts off high but slowly goes down the longer it encrypts.
The problem is, this isn't the first time I've encrypted this particular HDD, after another issue involving VeraCrypt that was never solved, forcing me to reformat the whole drive and removing VeraCrypt all together. I decided to reinstall VeraCrypt on the drive and encrypt it again. It never took this long when I first encrypted the drive and I don't remember the speeds dropping either. I tried changing cables and while it did increase the speeds, I'm still seeing the speeds drop the longer its encrypting.
So I'm wondering if this is normal?
It actually is kind of common for HDDs to become slower, the longer they are in heavy use. The reason for this is that the drive becoms warmer over time. But for myself, I never saw a recent HDD dropping to 11 MiB/s though. Have a look at S.M.A.R.T. values and the temperature of the drive. Maybe this already nails down the reason for the speed drop. For this you would have to put the drive out of the case into the computer. Only do this if you know what you are doing. Also have a look at CPU usage as this could also be a reason for speed drops.
I assume, both your computer and your HDD case support USB 3+, right?
Greets
Now that you mention it the temps were pretty high, like 120 something degrees to where CrystalDisk was giving me a yellow warning. Like I mentioned earlier, I don't remember it taking this long or getting this hot the first time I encrypted this drive. It's just a normal external HDD that plugs into a laptop's USB ports, both of which have USB 3.0. I've tried using different USB ports on the laptop but still the same results.
The CPU was normal, not even touching 50%. It's a Intel based laptop using a Intel Pentium J3710 @ 1.60GHz CPU. It never took more than a day to fully encrypt something but I wish I could remember the speeds.
As mentioned before, high temps will lower speeds. Most USB drives have cooling issues for sure. This would also affect the CPU (have a look at its temps, too). Additionally, have a closer look at CPU usage while encrypting - if one core is completely busy, it is likely the most you can get out of the system at this point in time.
Another thing you could try would be to pause the encryption process, let everything cool down for a while and the resume the encryption. I bet it will get a speed up as everything cooled down and can operate at max performance again.
Greets
Sorry for the late reply but I ended up using quick format before I saw your last post. It's just not worth it, I'd have to stop it from overheating every hour and continue to constantly monitor it hourly. Sadly I wasn't able to see the CPU temps.
I just wanted to know why now all of a sudden, I never had this issue when I first encrypted this drive. I even encrypt another external drive just days before this topic and had no issue either.
Last edit: James 2021-09-16
I'm sorry that I'm not able to give you an answer from afar as I'm not familiar with your specific hardware circumstances. All I can tell is that a slow down over time is very common. That, in your case, the slow down is quite extreme, may have a reason but hardware specific issues are always hard to debug if one has no direct access to the hardware.
Greets