Really bad performance on my MacBook has been driving me up the wall the past few days. Closing all un-needed programs didn't help. I ran several Mac utilities on my system, rebooted and so on and so forth.
Still no good.
There's 8GB of RAM in my (latest model - fall 2010) white MacBook and that really sped things up when it replaced the stock 2GB. It was very frustrating.
Activity Monitor (in your Application/Utilities folder) didn't reveal anything interesting. Everything seemed slow, from opening a file with vi to browsing to a web page.
Searching for "slow mac" brought up references to pages loading slowly. And that reminded me of namebench, a program that tests the fastest DNS servers for you.
Picking the fastest DNS servers isn't as simple as using OpenDNS or Google's Public DNS because some sites like Netflix return different answers depending on where you are and what internet provider you're using. What's right for OpenDNS may be much slower for you.
After running namebench, it detected that 22.214.171.124, the DNS provided by Comcast, my ISP, was not working. Using the advice from namebench I put 3 different DNS providers into the router, including two other Comcast addresses and a Google Public DNS.
Today, it's like I have a new machine it's so fast. Opening files is faster, browsing web pages is much faster, etc., etc.
Because of things like Dropbox or other background processes running, it seems like those slowed everything down, even something like vi on a local file.
So if your machine seems inexplicably slow, try using namebench and follow its advice.