The "thermodynamics" of the 19th century was mostly about static
systems in thermodynamic equilibrium.
It was formulated to calculate the initial and final entropies
when a system evolved from one equilibrium state to another.
In this older view of thermodynamics (that would be better referred to as thermostatics) there is no direct relationship between a natural process (e.g. chemical reactions, conduction of heat) and the rate at which entropy changes.
During the 20th century, physicists like Onsager, Eckart (see The Thermodynamics of Irreversible Processes), Prigogine and others extended the formalism of thermostatics to link the rate of entropy
change to rates of the out-of-equilibrium processes (e.g., the Clausius–Duhem inequality).
From this modern angle, thermodynamics is a theory of irreversible processes, not just a theory of equilibrium states.
(Kondepudi & Prigogine, Modern Thermodynamics: From Heat Engines to Dissipative Structures).
Therefore, modern thermodynamics is equipped with a formalism to calculate the rate of entropy changes due to irreversible processes (see e.g. On the Irreversible Production of Entropy
by Tolman). Hence, thermodynamics describes macroscopic systems that vary slowly in time (slowly enough that we can consider a limited number of collective degrees of freedom, see e.g. section II of this paper for a contemporary point of view), not necessarily so slowly that everything is always at equilibrium.
Within thermodynamics, thermostatics deals with equilibrium states and transformations between them where time is not an explicit variable: thermostatics ignores the flows, i.e. the time derivatives of the macroscopic quantities such as energy, density, chemical fractions... and everything that happens during the evolution between the two equilibrium states.
As a final note, beware that terminology may differ a lot between different books, especially when it comes down to terms like "adiabatic" to indicate slow processes (so if you check any book, be sure of what they mean by certain key terms).