MBP performance very bad

Question

So I bought my MBP 13" this summer of 09. It's been less than 6 months, and my computer's performance has been pretty bad (since the beginning) compared to my white macbook that I bought in summer 08 and the white macbook that my dad bought afterwards with the nvidia 9400m graphics card.
What is my problem?
My problem is that the boot-up is extremely slow (compared to the macbooks). It was fine when I first got it, but just randomly it's giving trouble. I have itunes helper, growl, and stickies running on bootup (just like my old macbook).
2nd problem is that I am seeing the rainbow ball more often for simple processes, like opening safari or an application like itunes. When I play games like call of duty 4 (on the default optimal settings) it plays fine and then it randomly starts lagging and keeps lagging unless you shut down the game and restart it. At first I thought the lag occurred when there were too many activities going on, but it should stop lagging afterwards. Nope! it still lags. In fact, it just starts lagging randomly even if nothing heavy is going on (like i'm just walking to get to the next checkpoint). The same thing happens to me when I play Farcry 2 on my windows partition. It would play fine and then randomly it would lag horribly.
So what have I done?
First, I cleared some space in my disk (I have 160 gig hdd - 30 of which is for Windows XP) so that I had around 30 gigs free on my Mac's partition. It still did not solve the problem.
Next I backed everything up using Time-machine, and formatted my mac's partition and installed Snow-leopard 10.6.2 on it from Leopard 10.5.8.
Results:
Bootup still slow (tiny bit faster)
Game still lags randomly (and stays laggy even when things "calm" down)
Performance slightly better, but still not good.
Can anyone guide me to what might be the source of the problem and if possible a solution please?

Answer 1

One of the first and easiest diagnostics to perform if something has changed with your computer, and that is what you complaint is as far as I interpret it, is to log in as another User. Create a new User Account and do this. This lets you know if your problem is system wide or user wide.
Also, and first actually, Repair Permissions and Repair Disk (at least verify) using Disk Utility. It costs nothing and might solve your problem.
If you used to have enough RAM you still do as RAM does not diminish. This of course assumes you are not using application that are significantly more RAM intensive than you used to use.
Leaving 10% of your hard drive free is only required if your hard drive is 20GB in total size. In other words, the amount you need free is not a function of the total amount that exists, it is a function of what your computer needs for swap space and that is a function of the particular apps you are running at any given time. After a clean restart, launch Activity Monitor and click the System Memory tab. See how much green unused RAM there is. Note that there will be little if any page outs. Keep Activity Monitor open and position its window such that you can keep an eye on the page outs. Go about your computing activity and note if or under what circumstances the page outs increase in a significant way.
Ideally you want zero page outs but realistically a few, such as a very small percentage of the page ins is acceptable. The thing is, if your RAM is too little it will show up as excessive page outs. If they are few, and brief, then adding more RAM will only save you the time that it took for the page outs to be incurred.
Unless I missed it, you did not mention how long it takes your machine to boot. That would be very useful information. Also mention how long you recall it requiring back when it was faster.
My suspicion is that you have some unwanted application or process going on taking up CPU time. This can be discovered in Activity under %CPU "All Processes" not My Processes. Nothing should be using double digit percentage except the app you are running and that depends entirely on the particular app.
Unlike a Windows PC you do not have a registry that is constantly filling up with garbage. You do not have a disk that is constantly getting more fragmented and needs tending. You do not have anything that should be constantly getting worse and slowing your machine, unless your hard drive is very very near full. I have tested this a number of times and you can fill the drive to the brim before it gives any measurable slow down with the apps I routinely run. Any slow down problem on the Mac can almost always be discovered and remedied without re-installs or any other extreme measures.