I believe that threshold for CPU performance is currently around a low/mid-range i5. Getting a faster processor than that will not translate into any tangible benefits in everyday usage. Even if there were a CPU that had twice the performance of an i5, an end user can not tell the difference between "practically instantaneous" and "1/2 practically instantaneous."
Likewise, there are limits to storage capacity. My parents, for example, store everything on a single 1TB hard disk. Their drive barely has 250GB used, OS and all. At their current rate of consumption, they might exceed 50% disk use in 5 years. Now we could come up with cases where the computer user is a photographer, a videographer, or likes to collect linux isos, but I believe in most real "casual use" cases, anything >1TB (currently) may as well be infinite.
Obviously there are also upper limits on what screen size a casual user would be able to effectively utilize. We should note, of course, that many casual users do not understand how to effectively manage windows between two monitors. I think the coming increase in pixel density coupled with a medium (say, 21 inch) monitor would provide a casual user with about as much screen as they could effectively use.
Lastly, there are cost trade-offs for all these things. Consider home internet. It is true that power users might be able to make do with internet speeds that would frustrate casual users (e.g. using Opera Turbo or using a data plan.) However, in my area for example, you would have to pay roughly 3x as much to get a "performance" 25 Mbps package versus the entry level 3-5 Mbps. Would it make sense for someone to pay that much more for internet just so that their youtube videos buffered faster?
Man this is slow: "top -ores"
"Hey hon, can I close any of these 4 word documents? What about the 15 webpages? No? Okay, I'll go get my laptop."