view your hdd like your bedroom. partitioning is like cleaning your room = less defragmentation over smaller parts = easier to find stuff = seek times faster.
one fat drive is like a messy bedroom, that sock you want/ under the bed. more areas for fragmentation = seek time longer.
also to note, heavy fragmentation will also reduce data transfer rates as the heads need to move to get to the data you want, more movement = less data being transfered per second.
so yes, imo partitioning does help keep your pc performing optimally. you wont 'gain' speed, but you wont LOSE any due to large fragmentation.