Restoring flash card write speed

I just realized, that one of my flash cards has a very varying write speed. Some times 6-8 MB/s (which is not impressive for a SanDisk Class 10 that should write with at least 10 MB/s https://www.sdcard.org/developers/overview/speed_class/ ) but I have seen it down to 600 KB/s.

Update: I have written a post about my experience and research on microSD cards - don't buy to cheap .

I read on http://www.thriveforums.org/forum/toshiba-thrive-general-discussions/15040-sd-card-issues-problems-solutions.html that it might be due to file system fragmentation and a normal formatting helped. It did not. Now I got the around 600 MB/s some times, where the lowest speed before was 1.3 MB/s.

Has anyone knowledge in this area? The card is around 6 months old and have taken around 150.000 images + a little video which for the 64 GB card sums to around 15 total overwrites. Should I try to RMA it? Two of my other cards are still useable but only lets me write with 4 MB/S. Same type of card, same use but no video.

PS. My card reader is a Lexar, that reads with up to 80 MB/s from these cards and should do more for faster cards.

Could this problem also be the reason for your problem with shooting att 2 images per second?

http://forum.mapillary.com/t/xiaomi-yi-doing-1-pic-0-5-without-hacks/415/2?u=harry

Definitely a possibility. In that test I used that card that at least got 4 MB/s and not the real bad one.

I am currenty looking into the trim system command ( http://haydenjames.io/increase-performance-lifespan-ssds-sd-cards/ ) which is called ERASE for SD cards. But it does not seem like this card supports any of them:

sudo hdparm -I /dev/sdd

/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
Standards:
Likely used: 1
Configuration:
Logical max current
cylinders 0 0
heads 0 0
sectors/track 0 0

Logical/Physical Sector size: 512 bytes
device size with M = 10241024: 0 MBytes
device size with M = 1000
1000: 0 MBytes
cache/buffer size = unknown
Capabilities:
IORDY not likely
Cannot perform double-word IO
R/W multiple sector transfer: not supported
DMA: not supported
PIO: pio0

So I am looking for an utility that will fix the internal fragmentation of the card. I think that is the problem. If I find out more, I will do a complete writeup!