Help! Why is my 2011 i7 Macbook Pro so slow?

Greetings, all. First ever post here. A bit long-winded so I apologize in advance.

I have a 2011 15-inch Macbook Pro 8,2 with a 2.2ghz Core i7, upgraded to 16GB Corsair Vengeance ram and a 1TB Seagate fusion hard drive (99GB free space). Running Yosemite 10.10.3. I'm in the beta program but have not installed the 10.10.4 beta software as of yet, so this is a stable release. In theory, this is still a beast even 4 years later - but as of late, I've been having some issues. If it tells you anything, my ORIGINAL 2006 Macbook Pro with a 32-bit Intel Core Duo (not even a Core 2) and 2GB ram seems to outrun it at times.

Startup takes between 3-4 minutes, dashboard widgets take between 45 seconds and a minute to display any information, applications take longer than normal to start, and Safari is having major issues. Safari is virtually unusable about 75% of the time. In fact, I had originally written this post in Safari and it became completely unresponsive when trying to post this. This happens often across all webpages. Many pages do not load period, and when they do, they load very slowly. When they do load, it's hit-or-miss as to how responsive they are. I have submitted this post from Chrome, which works much better, but I really do prefer Safari. Internet speed/router connection is not the issue, as I have other wireless devices that work just fine when this happens.

When performance is suffering, iStat Pro shows a processor load between 5-10%, with 4-6GB of the 16GB of ram being used - so this is obviously not an issue of not having enough processor power or ram, nor did I think it could be but I always check. The computer has been very well taken care of since day one and while it has been used often, it hasn't been used heavily. General use most of the time (web browsing, email, Facetime, word processing, etc). Light photo editing from time to time, and I rarely play games. When I do, it's an older flight simulator that I run on Windows 8.1 through boot camp. Strangely enough, Windows 8.1 runs flawlessly. I occasionally run a virtual machine (Virtualbox) as well. It runs the virtual machine just fine, though the computer does get hot, which is understandable and to be expected - but that brings me to my next point.

It also has periods with major temperature spikes - between 80 and 90 degrees celsius with no change of load or change of computer positioning, it just shoots up. Normally it hangs out between 40-50 degrees celsius. It's at 61 degrees right now and the only applications I have open are Chrome and a PDF in Preview.

I do have Kaspersky Antivirus installed, and it's possible it's partly to blame. However, I don't know how big a resource hog Kaspersky is. I know Norton is bad, but I don't know about Kaspersky.

Other than possibly the Antivirus, does anyone have any idea what could be causing these issues? Is there any maintenance I can/should do, whether it be on the hardware or software side? I'm fairly technologically inclined but I'm at a loss here because I haven't really done anything to my knowledge to cause these issues to happen. Any and all comments are welcome and appreciated.

Sound off.

Thanks in advance!