When I run Load Data Warehouse - Incremental, it takes very long time, intermittently. My Full Load runs quite fast. When we check DWH_GATHER_METRICS_V, we see most of the time is taken by DWH_CMN_MV_LOOKUP.
The intermittent slowness of DWH_CMN_MV_LOOKUP on Incremental Load was addressed by Development in user story S150853 in PPM version 15.4.1.
To get this issue addressed, plan to upgrade to 15.4.1 or higher (it's best to stay on the most current version).
When you upgrade, the Load Data Warehouse Incremental Load will no longer take that much time on DWH_CMN_MV_LOOKUP.
Until you upgrade, what you could do is schedule a Full Load to run instead, few times per day.