Downgrade OS Due To Slow Mac After Upgrade

I have a 20 inch iMac. It's years old. Originally it had Snow Leopard, now it's updated to Yosemite.

Since the update to Yosemite, my Mac is extremely slow. It's non response to clicks for 10-20 seconds.

I did everything possible to make it fast but nothing has worked so far.

Finally, I realized that Apple should not have let old Mac's update to their latest OS because it makes old Mac's **** slow.

I want to do the obvious thing - downgrade it back to the original OS - Snow Leopard.

BUT...

Like most Mac's my DVD drive had gone corrupt just in a year. So despite of having a DVD of Snow Leopard, I cannot install it.

Please help me with some option through which I can install the old OS in my iMac to make it usable again.