From this thread:
Tomtom 6 real slow with treo 680
"Very important is the cluster size of your card. You will get the best results with the following cluster sizes. Use the DOS box for formatting your card. I`m using a Transcend 4 GB / 150x with best results (16K), even with big Western Europe map.
Less than 4 GB (FAT16):
2 GB SD (SanDisk) FAT16 mit 32K: format X: /FS:FAT /A:32K
4 GB and more (FAT32):
4 GB SD or SDHC (Transcend) FAT32 mit 16K: format X: /FS:FAT32 /A:16K
"X" is the drive letter."