>Does hub 1 automatically advertise the routes it learns from hub 2 to hubs spokes that are not peering with hub 2?
Correct.
>what will be the impact on traffic to my spoke 1 peering with hub1, if I have a spoke 2 advertising a specific route to Hub 2 which again hub advertises automatically to HUB 1
If spoke1 wanted to talk to spoke 2, spoke 1 would send traffic to hub1, which would then send it to hub2, and then onto spoke2.