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@Gordon
I'm afraid I think this is just the nature of most filesystems. As I understand it, FAT writes in blocks, but there's a central table that contains information about the file. While writing a block takes a fixed amount of time, the filesystem will occasionally have to update the FAT table (I think?) which will involve a read, erase and write, and it definitely has to search around to find the next free block to write.
The SD card library itself is in 'low memory' mode, and I guess potentially moving out of there into the mode where it uses up more RAM might help with the speed issues, but I'd imagine that there will still be times when it takes a long time.
The problem has been that the filesystem lib used (iirc?) 2k of RAM all the time, whether it was used or not. It's something that people who don't use SD cards would get fed up with. Potentially now it could be modified to dynamically allocate that memory, but I think that's quite a bit of work.