It could take up to 10 seconds to create replication slot.
In addition to that when replica fails to connect to the master via
streaming replication it doesn't retry immediately, but with some
timeout (5 seconds). 10 + 5 == 15 what causes replication check
scenarios fail.
According to https://github.com/zalando/patroni/issues/48 'clonefrom'
tag should be boolean and it should be used to mark node as a suitable
for creation of a new replica from. If there are more then one such node
in the cluster (with tag clonefrom=true), one of them will be chosed
randomly.
Alexander Kukushkin