change encoder speed in gdk-pixbuf plug-in (#1191)

diff --git a/contrib/gdk-pixbuf/loader.c b/contrib/gdk-pixbuf/loader.c
index 32c6eee..28e6e4e 100644
--- a/contrib/gdk-pixbuf/loader.c
+++ b/contrib/gdk-pixbuf/loader.c
@@ -466,7 +466,7 @@
     encoder->maxQuantizer = max_quantizer;
     encoder->minQuantizerAlpha = 0;
     encoder->maxQuantizerAlpha = alpha_quantizer;
-    encoder->speed = 7;
+    encoder->speed = 6;
 
     res = avifEncoderWrite(encoder, avif, &raw);
     avifEncoderDestroy(encoder);