SanDisk SD Card, 2 GB, Class 2, becomes slow

I have the following problem with a Win CE Device

If i format the card with extFat and Standard Blocksize i can copy my tesdata( 122 MB 3081 Files 34 Folders ) in 1:30 min

If i delete and copy the test data again in some cycles the time for copying the testdata increases up to 20 minutes.

After reformatting ( no fast format ) the copy of the testdata is fast again (1:30 min)

Though the device with the SD card should run in non-stop operation where it is not possible to reformat the card to get it fast again.

The same problem also occeurs on a PC with the SunDisk MicroMate reader.
It has been tested with 6 equal cards. They all show the Phenomen.

How can this Problem be explained and how can it be solved?

Also to be mentioned:
The tool H2Testw shows always the same reading- and writingspeed ( writing: 7.89 MByte/s reading 10.4 MByte/s ) no matter how  often the test is repeated. H2Testw uses 1GB Testfiles.
Also one card wasn’t to get fast again with reformatting. It always takes at about 20 minutes to copy the testdata. H2Testw shows a writing speed of 2.89 MByte/s for this card.