What is causing Unity to be slow on startup?

This is the same problem i "WAS" facing but solved it, seems like a startup application was causing that problem.

Ok here is what i did -

sudo sed -i 's/NoDisplay=true/NoDisplay=false/g' /etc/xdg/autostart/*.desktop

open startup application then uncheck - DISK NOTIFICATION (The Disk Utility notification is used to report disk failures using the SMART predictive technology). It explains your quote also

I noticed more disc activity than usual

if u like u can also shutoff (it depends on your needs) -

AT-SPI D-BUS BUS, Backup Monitor, Check for new hardware drivers, Desktop Sharing, Onboard, Ocra Screen Reader, Personal file sharing,

Before unity was taking near approx. about 20 seconds to load now it takes 3 seconds roughly.

What I would do for that is to open a console (or two) while the system starts with Ctrl+Alt+F1/F2 then use top in conjunction with iostat -x (from sysstat package) to determine what's happen.

Other useful tools you may need some time for that kind of problem are those from inotify-tools package : inotifywait and inotifywatch which permits to monitor files accesses.