diff --git a/src/PlatformBackends/X11ImageGrabber.cpp b/src/PlatformBackends/X11ImageGrabber.cpp
--- a/src/PlatformBackends/X11ImageGrabber.cpp
+++ b/src/PlatformBackends/X11ImageGrabber.cpp
@@ -194,8 +194,22 @@
     // The RGB32 format requires data format 0xffRRGGBB, ensure that this fourth byte really is 0xff
     if (format == QImage::Format_RGB32) {
         quint32 *data = reinterpret_cast<quint32 *>(xcbImage->data);
-        for (int i = 0; i < xcbImage->width * xcbImage->height; i++) {
-            data[i] |= 0xff000000;
+
+        const auto imageSize = xcbImage->width * xcbImage->height;
+
+        bool sanitationNeeded = false;
+        // Check the first 1000 pixels. If they don't have any bogus alpha values, assume the entire image to be sane.
+        for (int i = 0; i < qMin(1000, imageSize); ++i) {
+            if (data[i] != (data[i] | 0xff000000)) {
+                sanitationNeeded = true;
+                break;
+            }
+        }
+
+        if (sanitationNeeded) {
+            for (int i = 0; i < imageSize; ++i) {
+                data[i] |= 0xff000000;
+            }
         }
     }