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Update #3 (reply to Tom's comments)
Here is what I entered in Mount Options Filesystem Type and what is type is displayed in System Monitor:

I managed to mount both external SSDs (SanDisk and Samsung) with ntfs3. So, SanDisk is connected to USB 2.0 and Samsung -- to USB 3.0. I copy files from SanDisk to Samsung. System Monitor shows writing speed jumping back and forth from 1.5 MB/s to 40 MB/s.

Update #2 (reply to Tom's and Cas' comments)
lsblk -f output:

NAME FSTYPE FSVER LABEL        UUID                                 FSAVAIL FSUSE% MOUNTPOINTS
sda                                                                                
├─sda1
│                                                                                  
├─sda2
│    ext4   1.0                83e3f693-2043-406e-88ce-b989162770f4  162.3G     6% /
├─sda3
│    swap   1                  4cae0365-911f-424f-9136-1bb25525b16d                [SWAP]
└─sda5
     ntfs         SSD_Volume   12E7C9690C5CA552                       84.9G    69% /mnt/SSD_Volume
sdb                                                                                
├─sdb1
│                                                                                  
├─sdb2
│    ntfs         Internal_HDD A6505DCE505DA639                       12.3G    99% /mnt/Internal_HDD
└─sdb5
     ntfs         HDD_Small    33D3C68F4222B7C3                      488.9M    11% /mnt/HDD_Small
sdc                                                                                
└─sdc1
     ntfs         SanDisk_4TB  108BFDA749FC5E38                        1.5T    60% /mnt/SanDisk_4TB
sdd                                                                                
└─sdd1
     ntfs         Samsung_4TB  6B666CCA02B40C8D                        3.4T     5% /mnt/Samsung_4TB

sudo blkid output:

/dev/sdd1: LABEL="Samsung_4TB" BLOCK_SIZE="512" UUID="6B666CCA02B40C8D" TYPE="ntfs" PARTUUID="8178173d-d087-438c-813e-1862da33d89b"
/dev/sdb2: LABEL="Internal_HDD" BLOCK_SIZE="512" UUID="A6505DCE505DA639" TYPE="ntfs" PARTUUID="e5b5b7a8-02"
/dev/sdb5: LABEL="HDD_Small" BLOCK_SIZE="512" UUID="33D3C68F4222B7C3" TYPE="ntfs" PARTUUID="e5b5b7a8-05"
/dev/sdc1: LABEL="SanDisk_4TB" BLOCK_SIZE="512" UUID="108BFDA749FC5E38" TYPE="ntfs" PARTUUID="94cd78f9-e299-4109-b967-0d7601ac376d"
/dev/sda2: UUID="83e3f693-2043-406e-88ce-b989162770f4" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="b17e5783-b225-44b6-bd7f-3b8b87015429"
/dev/sda5: LABEL="SSD_Volume" BLOCK_SIZE="512" UUID="12E7C9690C5CA552" TYPE="ntfs" PARTUUID="49c6d460-2269-4b53-8619-af83b3ffb031"
/dev/sda3: UUID="4cae0365-911f-424f-9136-1bb25525b16d" TYPE="swap" PARTUUID="239cd834-a356-4291-8978-3044b19fda9a"
/dev/sda1: PARTUUID="ce85a18d-7fb5-4f71-b808-bd974bef09b7"

Update (reply to Tom's comments)
The system partition (which is fast) is ext4. The rest are NTFS (System Monitor names their type as fuseblk).

sudo dmesg | grep -i ses gives:

[ 3955.118333] scsi 7:0:0:1: Enclosure         SanDisk  SES Device       8055 PQ: 0 ANSI: 6
[ 3955.121837] ses 7:0:0:1: Attached Enclosure device
[ 3955.121981] ses 7:0:0:1: Attached scsi generic sg3 type 13
[ 3955.122124] ses 7:0:0:1: Failed to get diagnostic page 0x1
[ 3955.122130] ses 7:0:0:1: Failed to bind enclosure -19

I'm copying files from SanDisk (sdc) to Samsung (sdd).

iostat 5 5 -N outputs the reading and writing speeds which look consistent:

           1.69    0.00    0.46    5.95    0.00   91.90

Device             tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd
sdc               3.80      1638.40         0.00         0.00       8192          0          0
sdd               3.60         0.00      1843.20         0.00          0       9216          0

I also tested copying files from the system partition (ext4) and the writing speed is still about 1.5 MB/s most of the time but sometimes it reaches 100-150 MB/s for short periods of time and drops again to 1.5 MB/s.