Freya is kind of sluggish on my Mac mini late 2009: core 2 duo, 8 GB of ram, SSD disk & gforce 9400. I am experiencing a ~1 sec delay to focus windows and when I click on apps buttons. Also the wing panel is slow as well.
My guess that it is something with Gala, because I don't have this issues with Xubuntu 14.04 (same base as Freya).
Any ideas ? Should I wait for the new hardware enablement stack ? When it will came out ?
This is most likely due to your video card driver. If you are using the open source video driver that installs with freya by default (noveau), I and many others have experienced quite a few issues with that driver. You should try installing the proprietary nvidia drivers and see if that fixes your issue. Personally I've had the best luck with the 331 version of the nvidia driver. You can install it by running sudo apt-get install nvidia-331 nvidia-331-updates from the terminal and then rebooting your machine.
The 14.04.3 hardware enablement stack should be installable in August, and may provide a resolution to your issue. Alternatively, your graphics card may be using a sub-optimal driver. Consider using Additional Drivers to select an alternative and check for any improvement (this will likely require a reboot).