There are various techniques which helps you to speed up Mac. One of them is defragmentation, which helps to keep scattered data in organise form in Mac memory. Read here:- http://www.websigmas.com/web-tech/how-to-defrag-free-space-of-mac-drive/

An I/O error usually indicates a bad drive or failing controller/cable. You should try returning the drive for a replacement. I would like to suggest you to do partition of your Mac drive by using Stellar partition Manager, it has great features. Let me know.

With the passage of time, when we install, delete, modify applications and data on Mac, all these activities raises the chances of fragmentation. I think you should Defrag Mac drive with the product of stellar. It helps to keep all fragmented data together in memory and increase the speed of Mac.

You can make your Mac faster. You can go through below link. With the passage of time Mac gets slow because of system junks, caches, language files, unwanted applications, duplicate files.

Hi, You should check your start up items and remove unwanted items. Another process you can do 'Clean up'. After a long uses , lots of caches, system junks, language files etc.