It makes very little, if any, difference as to whether an OS is the first partition, or indeed the last partition, on a HDD. It does take slightly longer to boot up, but that's because the system has to display the available OSes and wait for a response from you as to which you want to start. Note that the default (which is the last OS installed) will automatically start after 30s unless you select another OS, in which case you will need to press enter/return in order to start. You can also press enter/return to start the default OS, just to shave a few seconds off the boot time. Note that the default OS is the only one that will start automatically. Personally, I find that 30s is an excessive amount of time, and I change this to 10s. This reduces the boot up time considerably, yet still gives me sufficient time to choose an alternative OS.
I have not noticed any significant differences in speed between running a single OS (as I used to) or running any of my OSes currently installed.