If you parition and store your data on the rigt paritions it actually does speed up your drive. You are right, copying partition to partition is no faster but saying partitions makes your drive slower isn't right. It's possible to partition your drive in a way that you can store your most used files on the fastest part of the disk and improve performance.Partitioning a drive doesn't improve performace with modern drives. In fact it will reduce performance on the drive since the heads will have to move between partitions. Coping data from one partition to another in particular takes a big performance hit.
If you create a partition at the outer/leading edge of your drive (*1), and install your operating system & applications there .. and use the inner/slower parts of the disk for storing files that don't require access during normal system operating (i.e. downloads, drivers, back-ups, Ghost images, etc.) .. you'll limit/restrict your drive's seeks to the fastest part of the drive.