I have a 2010 MacBook Pro. When I first got it, I was amazed by its speediness. Just a great computer. But time had taken its toll. Recently it lagged at every opportunity. Took forever to start up and to switch applications. Constantly got hung up. It was almost unusuble, and was having serious effects on my productivity--it is amazing how having to wait five seconds to switch to my browser window could make me forget what I had been trying to do. So I was planning to get a new computer this month.
But I am lazy and cheap, so I asked my IT-manager buddy if there was anything I could do in the interim to speed things up. He had some recommendations, such as disabling any background apps that I wasn't using, but he also had a more radical proposal: open the machine up, double the ram, and replace the hard drive with a solid-state drive.
We made it happen. All told, it was $285 (though I think that might be a discounted price because my buddy works for a university). And the difference is amazing. The computer starts up (from shut down) in about ten seconds. From sleep it is basically instantaneous. Opening and closing applications is a snap of the fingers. I couldn't be happier. The extra ram helps, but I think this is mostly the SSD.
Now I don't have any need for a new computer, so I probably saved at least $1000 over what I had been planning to spend.
The downside is that I'm told that this kind of upgrade is not possible for more recent Apple laptops. But if you've got an old MacBook, I'd definitely recommend this kind of upgrade.
Although in your case, Bart, I suspect it's the hard drive that made the biggest difference. Going SSD is a huge change. It's why processor specs are only a small part of the story on something like a MacBook Air.