MB-8505: tune xperf config

Change-Id: I0245213d9aa89c3cc41d33f15b851f6b0b0b7157
Reviewed-on: http://review.couchbase.org/27394
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>