I would suggest visiting your local friendly Apple Store/Genius Bar to deal with your performance issues.
As a prophyatic measure, what is your computing temperature? You can download the free iStat Pro https://www.apple.com/downloads/dashboard/status/istatpro.html which monitors the temperatures on your Dashboard. I don't know if this problem related to temperature, but Retina machines do run hotter than, say, a MBP.
Also, does this problem relate exclusively to online use, or is it wider than that? Specifically, under what conditions does it happen?
I look forward to your response.
Thanks for taking an interest and offering to help. Unfortunately, my Mac doesn't seem to be able to download any of the versions of istat citing that safari can't find the server - no luck using chrome either. Safari performance does improve with restarting the machine, however, I really don't want to restart my machine every time I use it; I had hoped that I could open my machine from sleep after using it the previous day without any issues. Is this a reasonable assumption? Occasionally I experience poor performance when using my work PC if I hibernate or log off rather than shut down, however, I expected better from my Mac.
Furthermore, Kaspersky have just informed me that the error messages I receive when my security database fails to update indicates that there is a problem with my broadband connection. However, as I mentioned earlier, I have 100MBit broadband and experience no issues when using my work PC, iPad or iPhone.
I'm guessing I'll have to follow your initial piece of advice and visit the Genius Bar to have them resolve the issue...