[gnome-shell] blur-effect: Add some more documentation about performance

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit b4d491a4d21f97b7a11a1eaca853d71921916c8f
Author: Jonas Dreßler <verdre v0yd nl>
Date:   Mon Feb 10 15:21:29 2020 +0100

    blur-effect: Add some more documentation about performance
    
    Explicitly mention that this effect works best for large blur radii and
    can be slow if the blurred actor/image is large and not being scaled.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/991

 src/shell-blur-effect.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/src/shell-blur-effect.c b/src/shell-blur-effect.c
index 4a2f8f782f..7f1be6b1df 100644
--- a/src/shell-blur-effect.c
+++ b/src/shell-blur-effect.c
@@ -42,7 +42,9 @@
  * # Optimizations
  *
  * There are a number of optimizations in place to make this blur implementation
- * real-time.
+ * real-time. All in all, the implementation performs best when using large
+ * blur-radii that allow downscaling the texture to smaller sizes, at small
+ * radii where no downscaling is possible this can easily halve the framerate.
  *
  * ## Multipass
  *