diff --git a/reader/MarCCD.cpp b/reader/MarCCD.cpp
index 114cf9a16..4f1ab03f5 100644
--- a/reader/MarCCD.cpp
+++ b/reader/MarCCD.cpp
@@ -306,7 +306,9 @@ Header ReadRaw(const std::string &path, uint16_t *out, size_t capacity) {
         throw JFJochException(JFJochExceptionCategory::ArrayOutOfBounds,
                               path + ": image is larger than the buffer given for it");
 
-    TIFF *tiff = TIFFOpen(path.c_str(), "r");
+    // "m": read, not memory-mapped. On macOS a mapped 32 MB frame is paged in under the copy out of it, as
+    // page faults (a fifth of the image passes' busy CPU samples on 4nwv); the same bits either way.
+    TIFF *tiff = TIFFOpen(path.c_str(), "rm");
     if (tiff == nullptr)
         throw JFJochException(JFJochExceptionCategory::TIFFGeneratorError, "Cannot open TIFF " + path);
     struct Closer {