OK, I've been having this problem for a while now, and the more I get into it, the more I think it's crazy. Now, I'm not sure this is a Windows 10 problem, it could also be the motherboard, I'm quite sure though that it's not a HDD problem. Anyway, here I go.

I'm running Windows 10 on an SSD, this drive runs fine.

I have another new HD in my computer, a 1TB Seagate Desktop HDD ST1000DM003. Now, after I wake the PC from sleep the read / write speed of the HD roughly halves. Everytime the PC goes slower and slower until you can't even watch a 1080 mp4 without massive lagging.

I have used CrystalDiskMark 5 for performance testing:

partition read (MB/s) write (MB/s)
c: [SSD] 543 510
c: (after sleep 1) 540 506
c: (after sleep 2) 539 508
d: [HDD 1] 122 114
d: (after sleep 1) 73 65
d: (after sleep 2) 32 29
e: [HDD 1] 95 95
e: (after sleep 1) 68 58
e: (after sleep 2) 32 29

Now, I guess this looks like it could also be a HDD problem (even though the HDD's new). But look what happens as soon as I connect another (dying) HDD:

partition read write
c: [SSD] 524 471
c: (after sleep 1) 538 507
c: (after sleep 2) 542 506
d: [HDD 1] 145 133
d: (after sleep 1) 122 112
d: (after sleep 2) 117 113
e: [HDD 1] 112 119
e: (after sleep 1) 90 94
e: (after sleep 2) 98 90
f: [HDD 2] 62 68
f: (after sleep 1) x x
f: (after sleep 2) x x
g: [HDD 2] 53 83
g: (after sleep 1) x x
g: (after sleep 2) x x

x = Benchmark didn't even start.

Now, obviously the problem doesn't appear as soon as another drive was connected. I'm absolutely clueless.

Things I have tried (might have tried more, it's been so long I don't remember anymore):
- changed SATA cable
- changed connector on motherboard
- updates BIOS
- looked for HDD firmware (nothing found)
- tried disabling fast startup
- tried disabling superfetc
- tried disabling Windows 10 notifications.