Slow computer with new SSD and prohibitory sign at boot

Hi,

A bit of back story:

A few weeks ago I suddenly encountered a 'folder questions mark' sign when turning on my MacBook Pro, indicating the computer can't find the hard drive. To cut a long story short and after much deliberation, it appears that my old hard drive was broken and so I simply upgraded to a Samsung 500GB SSD, which is now up and running Yosemite. I haven't yet looked at transferring any of my files from my old hard drive (if they can be recovered).

So the problem is two-fold;

First, my computer is now running slower than it was before (similar to what I've seen on computers with a hard drive rammed full), which I am surprised at given a new and almost completely empty SSD.

Secondly, when I start up my computer now, a prohibitory/stop sign comes up for the first 20 seconds or so, but then continues to load as normal.

I'm wondering if the slowness has anything to do with the partition I introduced on the SSD when I first booted with it; there's just one partition using basically the whole thing, but this might not be related. Has anyone experienced anything similar, or have any suggestions for either problem?

Thanks for any help.

MacBook Pro 13" Mid-2012

OS X Yosemite (10.10.3)

Processor 2.5 GHz Intel Core i5

Memory 4 GB 1600 MHz DDR3