  The Fast Multipole Method (FMM) is well known to possess a bottleneck arising
from decreasing workload on higher levels of the FMM tree [Greengard and Gropp,
Comp. Math. Appl., 20(7), 1990]. We show that this potential bottleneck can be
eliminated by overlapping multipole and local expansion computations with
direct kernel evaluations on the finest level grid.
