Here where I leave the highest internet speed is 10 mps so even if it slows down 20% for me it would be to much
As long as the connection is made at very high speed.. it is unlikely to have any great effect on your internet speed.
Since your internet is pretty slow it just means the airport that extends has to get a strong signal from the main airport.. so it can extend without causing too much loss.
It also depends of course on which model airport extreme you are using. But let me illustrate with pretty average sort of setup.
I put an older Gen4 Extreme to extend AC model Time Capsule. You can discover the actual link speed by floating your mouse cursor over the word Unknown where it shows connection.
As you can see the link is very good.. it is a model setup and the two are very close.. so 450Mbps link speed.
Now you have to start doing calculations.. a computer for example linked to the Extreme at say 200Mbps can never achieve better than half that speed. So around 100Mbps.. in ideal world there would be no slow down at all over the double hop but worst case is half again.. so that takes us down to 50Mbps. This is still a lot higher than your 10Mbps internet speed so I think you will find downloads work pretty much the same.
What will happen is increased latency. Due to the way packets are handled over two wireless routers.. the increase in latency can be so severe as to make things like voip calls impossible. So for purely data links it can work fine.. for real time links where you need the best possible latency you will have issues.. eg gaming will be a problem for fast action, and any phone calls, skype etc will be much worse. In those cases you really need to run ethernet backbone so each wireless access point stands independent of the others.