In a modern computer, lag occurs for a few simple reasons.
1. Not enough physical memory. Memory is basically pretty fast these days, but if you don't have enough to keep needed things in memory long-term, the system has to work like hell to swap stuff you need out to make room or in when you need it in memory. This is called "memory thrashing." It occurs because on a computer, EVERYTHING has to be in memory to be used.
2. If you have enough physical memory, you can have multiple things running at the same time, thus competing for the CPU as a resource. Even for multi-core CPU chips, you can only do so much at once before all of the CPU processing resources are booked solid. This comes about when you have too many things that run in the background. If you have a quad-core system and at least five active processes, you will start to see issues somewhere.
3. If your program has to do a lot of disk I/O, then competition for use of the disk will slow you down. This usually doesn't happen unless you have some sort of multi-tasking "thing" going on in the background on your system. If you have low physical memory, then the system swap manager will be using the disk pretty much all of the time and that task happens to have among the highest priorities on your system. There is an old saying: When your system swaps, your world stops.
4. After that, lags will occur based on devices that are failing. The error or event logs will be busy leaving behind device log entries complaining about stuff that is about to die. Not all devices die clean and hard. Some take their sweet, bloody time in dying, perhaps just to tease you.
5. If the problem is lagging on web pages, just remember that a chain is no stronger than its weakest link, and a network is no faster than its slowest hop. If your network card is on the sluggish side, you will end up waiting for everything. If you like to do something like MMORPG sessions, then contention / competition for bandwidth is probably significant.
It is rare, other than for hardware failures, for a modern display to be the cause of lag. BUT if you buy some oversized screen with something like 4086 x 3072 resolution and then try to play your ultra-complex graphics-oriented game on the maximum that the screen will support, THAT will slow you down significantly for most modern gaming cards. In this case, it would be overloading the graphics card's computational resources.