Partitioning will not affect hard drive speed.
If you are using FAT or FAT32, partitioning will allow you to reduce the sector size of your disk, which in turn means you will have more efficient storage on your drive.
For example, if your sector size on your drive is 16k, then any file that is 16k or less will take up 16k of space on your hard drive. In older OSes, the size of the hard drive limited your options when it came to choosing a sector size, so you would parition your drive into smaller chunks so that you could make smaller sector sizes. (I think 4k was the smallest sector size available)
These days with NTFS, sector size is basically irrelevant, so the main advantage of partitioning a drive is to allow you to organize your files better. I generally make a system partition (C drive), an apps partition (D drive and a data parition (E drive).