MB-100: speed limit for read hdgm case

Change-Id: I2e35cd250306bc4134bc24a23dbbb3d85e1937d1
Reviewed-on: http://review.couchbase.org/28003
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>