Maybe you experienced some temporary or local Github connectivity issues. And nobody except you can know what's going on in your local network.
20min for the download of ~100MB doesn't sound any fast either, but Eleutheria didn't mention any details of personal internet connection. Such a download time could happen when you only have something like 1MBit/s downstream.
Today I tried to download Sparrow Wallet again and the speed was improved but there were limits on download. There is definitely a problem on their side. I'll try to contact them and let them know about this issue.
I tried download it just now, but it use my maximum internet speed. It's likely their server was overloaded, although there's small chance your ISP throttle your connection. Next time you experience this problem, you might want to use VPN and see whether you still experience slow download speed.