Always format SD cards using SD Formatter SD Memory Card Formatter for Windows/Mac | SD Association.
NEVER format SD card with OS utilities!
Here are some examples that show what can happen. First is an extreme case for determining free space on an SD. This type performance hit will also be true for seek.
Time to determine free space with a properly formatted card using the SdInfo sketch. See http://forum.arduino.cc/index.php?topic=228128.msg1648437#msg1648437 for more details on code.
Time 0.302 seconds.
millis: 302
freeClusters: 63883
freeSpace: 2093.32 MB (MB = 1,000,000 bytes)
Same card formatted FAT32 with small clusters on Windows.
34.885 seconds.
millis: 34885
freeClusters: 4033302
freeSpace: 2065.05 MB (MB = 1,000,000 bytes)
Here are results running the SdFat bench sketch on this card. Write takes about four times longer with FAT32 and small clusters.
Properly formatted card.
Type is FAT16
File size 5MB
Buffer size 100 bytes
Starting write test. Please wait up to a minute
Write 358.47 KB/sec
Maximum latency: 58888 usec, Minimum Latency: 84 usec, Avg Latency: 273 usecStarting read test. Please wait up to a minute
Read 369.19 KB/sec
Maximum latency: 1912 usec, Minimum Latency: 84 usec, Avg Latency: 265 usec
Same card formatted FAT32 with small clusters on Windows.
Type is FAT32
File size 5MB
Buffer size 100 bytes
Starting write test. Please wait up to a minute
Write 89.43 KB/sec
Maximum latency: 117636 usec, Minimum Latency: 84 usec, Avg Latency: 1113 usecStarting read test. Please wait up to a minute
Read 225.44 KB/sec
Maximum latency: 1920 usec, Minimum Latency: 80 usec, Avg Latency: 437 usec
These are extreme cases but I do often see this type problem so please use this formatter SD Memory Card Formatter for Windows/Mac | SD Association. It will always produce the optimum format for your card.