I have used blender for a while now and it worked perfectly, but when I opened it up today it's being very slow/laggy. When I look around in 3d view it takes forever to respond, and everything is either delayed or slow.
Mine does this sometimes when my graphics drivers are out of date. Unless you installed a new plugin or something new to it, I wouldn't blame Blender for this. It is probably most noticeable in Blender but I am willing to bet that your graphics card drivers need to be updated. If you have NVidia, they usually have an app installed on your machine that has an update driver button. The app is called "GeForce Experience". http://www.geforce.com/geforce-experience/download
As William suggests, updating the drivers for the video card could help, but there might be other causes, too. Running slow could also mean that you have other issues. For example, if you run an application which does not manage memory correctly, you might lose a small amount (say a couple of megabytes) of available memory at a time, and eventually, there is not enough memory to for Blender to work properly.
Another situation in which this might occur is if the Blender file you're trying to open is too large. One reason this might happen is if you have multiple images packed in the file.
Another possible cause is if you don't have enough memory because of other processes running in the background. I've experienced all of these situations, myself.
For a better answer, it would be helpful to have more details about your system. What version of Blender are you using? What are the specifications of your hardware—what kind of processor, what clock speed, how much RAM, what kind of video card, how much video card memory, what operating system?
Also, what other software might be running at the time? I sometimes have multiple instances of Blender running, when, for example, I am linking or appending contents from one file into another. And instead of a calculator widget, I often make use of a spreadsheet for calculations. If there are multiple programs running, closing some of them can improve performance. Sometimes, when starting a new project I'll have a couple of instances of Blender running, and also have a browser window open to the internet so I can search for reference material on line, and I'll have one or more images open in GIMP.
Finally, I sometimes leave my computer powered on for days or weeks at a time, especially if I'm working on a large project. The longer the computer has been continuously on, the slower Blender (and other programs) get, and I find the speed improves by orders of magnitude, not only in Blender, but in other applications as well, by the simple expedient of saving all my work in progress, shutting down the computer, and rebooting it.