Physical parallel devices are not much faster than sequential ones

It is proven that any (uniform) family of physical parallel devices, recognizing a language C? with time-complixity T_P(n), can be simulated by a (uniform) family of sequential devices with time-complexity $\text{T}{}_{\mathrm{P}}\text{(n)}{}^{\mathrm{<mtext>1</mtext><mtext>d</mtext>}}$ (d is a constant, depending on the technology, but not greater than 13).