Optimize condition for removing written data in BufWriter

crates/async-compression/src/generic/write/buf_writer.rs

@@ -136,7 +136,10 @@ impl BufWriter {
     ) -> Poll<io::Result<&mut [u8]>> {
         ready!(self.partial_flush_buf(poll_write))?;
 
-        if self.written > 0 {
+        // when the flushed data is larger than or equal to half of yet-to-be-flushed data,
+        // the copyback could use version of memcpy that do copies from the head of the buffer.
+        // Anything smaller than that, an overlap would happen that forces use of memmove.
+        if self.written >= (self.buffered / 3) || self.buffered == self.buf.len() {
             self.remove_written();
         }