if someone can't follow what you're saying, you tend to assume that they're stupid. I'm not going to claim that there's no such thing as a stupid person, because I've been stupid (two days ago, for that matter). That said, reasons for perceived differences in mental bandwidth:
- speaker assumes items in main memory
- speaker misjudges cost of cache miss/page fault
- listener is preoccupied/distracted
- listener is not interested
- speaker makes large mental leaps
- speaker is unclear or overly abstract
- either is egocentric
- either is stubborn
- either is currently low on available mental bandwidth
- speaker doesn't know what he's talking about