Thats not a lot of documents for the _changes to process.
Whats the longest time it takes to get a response back from _changes feed if you just hit the rest end point as a user through a browser?
Also in the _changes feed request do you see Couchbase server getting lots of keys GET()s?
if so you might want to leverage SG channel cache more.
channel_cache_max_length: (Default: 500)
Maximum number of entries maintained in cache per channel.
Did you change the default timeout settings for the replicator on CBL? 10 seconds seems a little aggressive on the time out.