I would not recommend using 64K clusters. The FATGen document that describes the FAT file system layout explicitly discourages against doing so:
Values that cause a cluster size greater than 32K bytes do not work properly; do not try to define one. There is a misconception that values greater than this are OK. Some versions of some systems allow 64K bytes per cluster value. Many application setup programs will not work correctly on such a FAT volume.
The best way to speed up your storage card is to choose a file system format with less overhead (like FAT32/4K or FAT32/8K), increase the file system cache and keep the storage card defragmented. All the above can be accomplished with
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