A lot of people suffer from problems as their computer gets older. Both Windows and Mac based computers have these problems. The reason why the computer is running a lot slower now than it did two or three years ago is because there have been numerous programs installed over time that are taxing the system and making it slow to a crawl. There are lot of programs out there designed to help remedy this problem and some of them do a good job. However the law of unintended consequences takes effect. These programs can do just as much harm as they can do good.
Some programs will actually monitor your processes and will shut down processes that it "thinks" you don't need any more. This can cause a whole host of problems especially when it kills a process that Photoshop needs to run. If you are going to run software like this it is a great idea to turn it off when you are using Photoshop. If you don't want to turn it off there may be an option to have it not monitor certain applications, in the case that this is not possible then turning it off just while you are working in Photoshop may be the best option.
Overall, just be careful when using those types of programs especially when you are noticing weird behavior from a filter or Photoshop in general.