  Modern 4-wave mixing spectroscopies are expensive to obtain experimentally
and computationally. In certain cases, the unfavorable scaling of quantum
dynamics problems can be improved using a generalized quantum master equation
(GQME) approach. However, the inclusion of multiple (light-matter) interactions
complicates the equation of motion and leads to seemingly unavoidable cubic
scaling in time. In this paper, we present a formulation that greatly
simplifies and reduces the computational cost of previous work that extended
the GQME framework to treat arbitrary numbers of quantum measurements.
Specifically, we remove the time derivatives of quantum correlation functions
from the modified Mori-Nakajima-Zwanzig framework by switching to a
discrete-convolution implementation inspired by the transfer-tensor approach.
We then demonstrate the method's capabilities by simulating 2D electronic
spectra for the excitation-energy-transfer dimer model. In our method, the
resolution of the data can be arbitrarily coarsened, especially along the $t_2$
axis, which mirrors how the data are obtained experimentally. Even in a modest
case, this demands $\mathcal{O}(10^3)$ fewer data points. We are further able
to decompose the spectra into 1-, 2-, and 3-time correlations, showing how and
when the system enters a Markovian regime where further measurements are
unnecessary to predict future spectra and the scaling becomes quadratic. This
offers the ability to generate long-time spectra using only short-time data,
enabling access to timescales previously beyond the reach of standard
methodologies.
