Investigate initial build time difference between 3K and 6k plasma (in-memory)

For 3K single node plasma (in-memory) initial build perf test, build time is 16 min.
For 6K 2-node plasma initial build perf test, build time is 26.8 min.

For the 6K test, each node has 3K indexes. There is no CPU or IO saturation in this test, so theoretically, the build time between the 3K and 6K should be able the same. For 3K test, mutation queue size is kept about 100-200K. For 6K test, mutation queue size is close to 0 for both nodes. For 3K test, DCP rate tops out at 175K/s, but for 6K test, DCP rate tops out at 120K. Projector CPU and RSS are about the same for both tests.

Note that the same build time discrepancies occur in MOI as well (9 min vs 26 min).