  In literature, the cost of a partitioned fluid-structure interaction scheme
is typically assessed by the number of coupling iterations required per time
step, while ignoring the internal iterations within the nonlinear subproblems.
In this work, we demonstrate that these internal iterations have a significant
influence on the computational cost of the coupled simulation. Particular
attention is paid to how limiting the number of iterations within each solver
call can shorten the overall run time, as it avoids polishing the subproblem
solution using unconverged coupling data. Based on systematic parameter
studies, we investigate the optimal number of subproblem iterations per
coupling step. Lastly, this work proposes a new convergence criterion for
coupled systems that is based on the residuals of the subproblems and therefore
does not require any additional convergence tolerance for the coupling loop.
