QED cascades in an intense electromagnetic field can occur if the dynamical quantum parameter $\ensuremath{\chi}$ of a seed electron, which in Compton units coincides with the electron proper acceleration, attains the order of unity. We derive a general expression for $\ensuremath{\chi}$ of an initially slow electron in an arbitrary electromagnetic field for a time range $t\ensuremath{\ll}1/\ensuremath{\omega}$, where $\ensuremath{\omega}$ is the field carrier frequency. Using this formula, we consider a special field configuration of multiple colliding focused laser beams and optimize it to provide cascade development at laser power below 10 PW and intensity of the order of ${10}^{23}\phantom{\rule{4.pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$. Such parameters of the beams will be obtained with a new generation of laser facilities, particularly the ELI Beamlines, in the coming years.