My MacBook gets slow

Do you have an external drive to boot your system up with? This could be an HD or thumb drive.

What I think is the issue here is your disk maybe encountering errors but to tell you'll need to boot off a different disk so you can check it with Apples Disk Utility from the booting drive. Try first to fix the permissions and then the disk its self. You may need to run it a few times to clear out all of the errors. If after 8 or so tries the errors don't clear the drive maybe to far gone.

As to what to replace it with getting an SSD of equal size will be expensive. While right now you appear not to be using that much (a little more than 20 GB). Are you planing on using more storage with files you haven't put back?

I like SSHD units (hybrid HD) as you gain the benefit of the SSD performance but also get the deep storage of a HD. But before we travel down that road lets see if your HD is recoverable.

Lastly, being a newer system your HD could be covered under an extended warranty but you'll need to bring it into an Apple Store to find out.