Separate performance numbers from AzulSCSI firmware README

Separate performance numbers from basic AzulSCSI firmware README, since they are different between platforms.

Performance.md

@@ -0,0 +1,17 @@
+Performance
+-----------
+Performance characteristics of AzulSCSI differ depending on the hardware version. 
+
+With AzulSCSI V1.1, and verbose log messages disabled (debug DIP switch OFF), expected SCSI performance is ~3.5 MB/s read and ~2 MB/s write.
+
+With AzulSCSI V1.0, and verbose log messages disabled (debug DIP switch OFF), expected SCSI performance is 2.4 MB/s read and 2 MB/s write.
+
+With AzulSCSI Mini (V1.0), and verbose log messages disabled (default, unless you enable it in azulscsi.ini), expected SCSI performance is 2.4 MB/s read and 2 MB/s write.
+
+Slow SD cards or a fragmented filesystem can slow down access.
+
+Seek performance is best if image files are contiguous.
+
+For ExFAT filesystem this relies on a file flag set by PC.
+Current versions of exfat-fuse on Linux have an [issue](https://github.com/relan/exfat/pull/101) that causes the files not to be marked contiguous even when they are.
+This is indicated by message `WARNING: file HD00_512.hda is not contiguous. This will increase read latency.` in the log.