I mainly use gnome-shell, but I fired up Unity on my Ubuntu 13.10 running Thinkpad T400 (~4 year old machine, core 2 duo) with 8 GB of RAM and a Samsung 840 pro

I mainly use gnome-shell, but I fired up Unity on my Ubuntu 13.10 running Thinkpad T400 (~4 year old machine, core 2 duo) with 8 GB of RAM and a Samsung 840 pro SSD. the unity dash comes up instantaneously for me; certainly less than 1/2 a second. I'm guessing the SSD makes a big difference, as it does with all other computer performance issues. A 128GB MLC SSD costs less than $100 these days, and using one as your OS drive is by far the most incredible performance improvement you can make on a desktop machine.