SanDisk Ultra 64 GB microSDXC has become very slow - internal fragmentation?

Last year I got 3 “SanDisk Ultra Android 64 GB microSDXC Class 10 Memory Card + SD Adapter up to 80 Mbps” and have used them in action cameras to take around 150.000 - 200.000 images on each. They have all much lower write speeds, and some times read speeds, now. One writes as slow as 600 KB/s (Class 10 should do at least 10 MB/s but this card should be much more). The two other cards will write as slow as 4 MB/s. All can write with 6-7 MB/s some times, but not enough for a Class 10.

My card reader is a fast (non Sandisk but a good brand) USB 3.0 reader, that at times reads these cards with 80 MB/s.

I have read that fragmentation can cause this slowness. Not file system framentation (as known from harddrives) but fragmentation due to the wear leveling algorithm. A fix should be to use a tool to issue a command that erases the card completely and lets the wear leveling use the space without fragmentation.

Does anybody know a tool that can do this?

On Linux I have tried to see if the commands (TRIM or ERASE) are supported, and they did not appear (see below). A tool for Windows is OK (I have a Windows 8 too).

sudo hdparm -I /dev/sdd

SG_IO: bad/missing sense data, sb[]: 70 00 05 00 00 00 00 0a 00 00 00 00 20 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

ATA device, with non-removable media
Likely used: 1
Logical max current
cylinders 0 0
heads 0 0
sectors/track 0 0

Logical/Physical Sector size: 512 bytes
device size with M = 1024*1024: 0 MBytes
device size with M = 1000*1000: 0 MBytes
cache/buffer size = unknown
IORDY not likely
Cannot perform double-word IO
R/W multiple sector transfer: not supported
DMA: not supported
PIO: pio0


what i would try is to deep format the card so disable the quick format option and then try the card again to see if its still very slow. this can be caused since the cards do not have a trim option to clean out the blocks of the card is can become slow with the time and the use of the card. 

so try to format the card and check out the results