Unreliable operation for some <Super> shortcuts

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I am using Ubuntu 14.04 with flashback metacity session.
I configured the "launch terminal" on the <Super>s shortcut. I also searched and replaced in dconf all other shortcuts with Super+s
However, the operation of this shortcut is extremely unreliable : sometimes it does not work at all, some other times it does work but only if i keep the Super key pressed significantly longer. More than that, in order to work i have to :
press super -> press s -> release s -> release super.
I also configured a Super+e shortcut for "home folder". This one works every time provided that the long sequence above is respected :
press super -> press e -> release e -> release super.

I would expect :
1.Super+s to work every time (it seems that something else may "consume" the event but i cannot find what it is, as nothing seems to happen visually when super+s does not launch the terminal.
 2. When the shortcuts are processed, i would expect the configured script/execution/action to be executed right after
press super-> press shortcut letter, WITHOUT requiring the long super press.

What is even more weird is that if i use flashback session with compiz, the shortcuts seem to always work and to work fast (without long super press).

DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=14.04
DISTRIB_CODENAME=trusty
DISTRIB_DESCRIPTION="Ubuntu 14.04.1 LTS"