MacBook Running Really Slow, Activity Monitor Reports HUGE VM
The first thing you might want to do is restart it. That will probably fix the problems. 

Other things you might want to check:


Do you have enough free disk space?
How much RAM do you have? Are you running any memory intensive application (or many)? I.e. you might have photoshop sitting idle, but ten 80mb RAW files open… that uses RAM, not CPU (as long as you’re not processing them in any way).
Are you running any sort of Virtualization software (VMware, Parallels, VirtualBox?) If yes, are they running a Virtual Machine?
Is your computer Paging too much to disk? (See activity Monitor, memory: Page ins vs. Page outs). A lot of Pageouts is bad (compared to Ins). That means your box is lacking physical RAM memory and must write to the (slow) HDD to save the memory pieces. This is one of the #1 causes for slowdowns. note: some page outs are ok, the system uses the swap no matter how much ram you have.
You already mention it, but sort by CPU Usage in All Processes (Activity Monitor) and make sure no process goes crazy every now and then. 
Open Terminal (/Applications/Utilities/Terminal.App) and execute:

ls -lh /var/vm


The output will consist of one (or more) swapfile0, 1, 2… and next to them you’ll see the size. Small sizes are ok (a few hundred Megabytes). If that swapfile is huge, then you have an Insufficient RAM problem. You need more RAM or less programs at the same time.

Here’s my output: (I have 12 GB of RAM, so that’s a lot to spare)



In the end, restart your computer and monitor those values throughout the day(s).