Add tree-sitter performance note

diff --git a/admin/notes/tree-sitter/performance b/admin/notes/tree-sitter/performance
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+TREE-SITTER PERFORMANCE NOTES -*- org -*-
+
+* Facts
+
+Incremental parsing of a few characters worth of edit usually takes
+less than 0.1ms. If it takes longer than that, something is wrong.  There’s one time where I found tree-sitter-c takes ~30ms to incremental parse. Updating to the latest version of tree-sitter-c solves it, so I didn’t investigate further.
+
+The ranges set for a parser doesn’t grow when you insert text into a
+range, so you have to update the ranges every time before
+parsing. Fortunately, changing ranges doesn’t invalidate incremental
+parsing, so there isn’t any performance lost in update ranges
+frequently.