Delta definitely spreads significantly faster/more efficiently than the original COVID (higher Ro). As such, you need higher levels of immunity to achieve herd immunity, and choke off wild transmission (1 - 1/Ro) - closer to 80% immunity for Delta, compared to 60% for the original strain.
The two mutations that characterize it make it spread more effectively, but are not related to severity or lethality.
It may be that the lower severity and lethality that we are seeing in the USA, is more an effect of the most vulnerable people being mostly removed from the cases, due to vaccine protection.
Delta was plenty lethal when it first shot through India, with low vaccination rates among the elderly.
Wish we had more and better data from India on Delta.
We know it had a brief wave, which is encouraging, but know nothing about deaths and so much more.
Hoping to see signs of a brief wave in UK soon.