The cpu in the router is able to process something like 3-4 MB/s
that's like 83 minutes or so.
It's due to the slow cpu and the lack of enough RAM memory these things take long.
The checking is because of 2 things usually:
1. to see what "blocks" are downloaded yet
2. to be sure all the downloaded "blocks" are downloaded correctly
to stop it from checking it all the time you should pause it before restarting
btw, 20GB is an awful lot for routers like these