For anyone having download problems, there is an easy fix for this.
Worked when season 4 was released and battlenet had the same downloadspeed issues
Works with season 5 release, personally tested.
Change version/region to Americas and use a VPN with an american location for example New York / New Jersey / Washington DC and download with 10-15-20 MB/s.
+1 to make this a sticky, as this problem seems to occur with every new season.