Typing speed lags on black Macbook

Question

I'm a fairly fast touch typist and I've noticed that when I start to really get into it, the typing starts to lag. This happens on every application I've tested it on including Terminal, Adium, TextEdit, Mail and Safari.
Here're the stats on my machine:
2.2 GHz Intel Core 2 Duo
4GB 667 MHz DDR2 SDRAM
Leopard 10.5.6 (fully updated)
With regards to System Preferences the Key Repeat Rate is at its maximum setting and the Delay Until Repeat is one notch away from maximum.
I upgraded the RAM (using Apple certified RAM from Corsair) and ran Apple's hardware check and even TechTool Deluxe, everything checked out fine. I don't experience any crashes or freezes at all which leads me to believe that the RAM is fine. The typeing lag happens even when I have only a few applications running such as Safari, Adium, Mail and iCal, and even when I just boot up the computer. Nothing process intensive in the background. iStat reports 86% idle CPU and 2.7GB free memory, so plenty to go round.
Some forums I've read suggessted resetting the PRAM so I've done this as well, but it hasn't helped. I've seen that others have this problem too but so far no solutions to resolve their issues. I'm wondering if anyone can help?
Many thanks.

Answer 1

Great question - I concur ~ mostly.
My white MacBook lagged the day it was taken out of the box. (this is factual as it caused problems when I created my dot MAC account)
Current Specs: OS 10.5.7, 4 GB RAM (upgraded from 1 GB when new), 2.2 GHz Intel Core 2.
The reason I added "mostly" above is that the problem has been greatly reduced, but I'm not sure what I did (or what I may have installed when the laptop was new and it lagged).
Generally I have done the following:
1. upgraded the RAM to 4 GB (as you did, but it was from Crucial).
1b. replaced a defective (flaky: intermittent) network card (Airport Card).
2. rid the computer of anything that uses Rosetta by using the Utility "Activity Monitor" and removing loaded software that uses Power PC rather than Intel. I had a few things that required Rosetta and have since stopped using those items.
3. I run all the software you listed with the _exception of Adium_. To be honest, I run without rebooting and never close anything which leads to many running applications and too many open windows. Everything, uses Intel and not Power PC though.
4. I suspect it will still happen if the laptop is busy at some task like aquiring a network that has problems, but I haven't recreated a test environment for that - yet.
A simple test that I tried after reading your post was to run my finger over the first row of keys which yielded - qwertyuiop. I've tried it slow and fast with the same results - good, no lagging.
Ramp it up with iTunes playing music and scanning (wirelessly) a document as OCR to text translation with all the other software running (most of it) - still qwertyuiop comes through correctly.
* I may not be hitting the keys as fast as you are, but I can certainly say that mine lagged for some unknown reason early on. Now, it does not, or I am unable to reproduce situations in which it does.
Is there a way to dictate processor priorities? (I am somewhat new to Apple)
Good Luck.
PS if I recall correctly, in the past another symptom that my laptop displayed was that it took too long to wake from sleep. Now, it wakes quickly. Of course, it takes a little longer to get into sleep with more RAM installed.