Why is an isothermal process slow?
It generally isn't. Any thermodynamic process would be slow IF the process is to be reversible.
A reversible process is simply one that is done infinitesimally slowly, so that there is 100% efficiency in the flow of energy from system to surroundings and vice versa.
In other words, the process would theoretically be done so slowly that the system has time to re-equilibrate after each disturbance during the process.
In reality, that never happens, but we can get close.