The first thing you probably did after rooting your Defy Mini was removing some factory apps like Cardio Trainer, Twitter, Facebook, Youtube and others...
Now I'll give you some tweaks you can apply to your Defy Mini to improve performance and make it more "usable", so let's start.
1) Get more CPU/GPU performance
-add the following lines to your buld.prop file ubicated in "/system/build.prop" (Mount partition as r/w) (using Root Explorer is the easiest way)
2) Setting interface rendering to Hardware (using gpu to draw interface)
-find the file "egl.cfg" ubicated in "/system/lib/egl", remove everything written on it and copy this "0 1 adreno200", it should look just like this:
-In the same dir "/system/lib/egl" find the file "libGLES_android.so" to "libGLES_android.bak", it should look just like this:
-RESTART YOUR DEVICE-
Test it for yourself, now your device will be able to play 480p videos with smoothness (25 - 30fps) (Adobe Flash Player for ARMv7, gpu video rendering, 16bit per pixel)