Just got an e-mail from them:
We sincerely apologize if you had problems downloading your REX4 purchase due to technical issues on our server.
We have since corrected these and have made available a newer updated version from a different server.
Please redownload your REX4 purchase using the link below.
There should be no problems now with download stability and speed.
Again we apologize and thank you
Too late...now what, not after I already made the second purchase from another store
Anyways the speeds are back to normal as they said.