Why is an isothermal process slow?

The majority of the time, it isn't. If a thermodynamic process were to be reversible, it would be slow.

Simply put, a reversible process is one that proceeds infinitesimally slowly, ensuring that energy flows from the system to its surroundings and vice versa with 100% efficiency.

Stated differently, the procedure would ideally be carried out at a slow enough pace to allow the system to recover from any disruptions it may encounter along the way.

Though it never actually occurs, we can come pretty close.

An isothermal process is slow because it occurs at a constant temperature, meaning there is no additional energy input to increase the rate of reaction or movement of particles. Without an increase in temperature, the kinetic energy of the molecules remains relatively constant, resulting in slower reaction rates and movement compared to processes that occur at higher temperatures.