What makes a computer slow down over time?

There are a lot of factors.
It can hardware; parts breaking down, fans getting clogged with dust and causing the CPU to throttle as it heats more and more, etc.

It can be software; viruses, file caches, disk getting filled (SSD and HDD), fragmented hard drive (HDD only), leftover program files, updates that don't run well on older hardware.

So there's generally not just one cause.
If you're having slow downs after installing Yosemite, then I suggest re-installing Yosemite. I haven't had any slow down. It actually feels a bit faster.

 

Youʻre right on both counts, and everything mentioned above.

 

-hard drives getting old or SSD's getting filled up and losing speed
-file/app caches
-lazy developers making their apps more and more inefficient
-fans getting dusty so they work harder and the CPU heats up faster and throttles

 

Installing MS Windows. Zing...

 

Also, the longer you have it, the more stuff you accumulate, apps and files. Clean it up regularly and keep a good portion of your HD free.

 

1. Little RAM. As you upgrade the OS it requires more and more RAM until you reach the minimum (which is very slow) and need to upgrade.

2. Dust. Over time dust builds up and your computer runs hotter. When the GPU is very hot, it will lower its clock speed to prevent overheating (thus making it laggy).

 

I must disagree a bit. RAM has little do with speed in the first place. It can cause slow-downs as your drive caches when there isn't enough free RAM, but OS upgrades should not raise RAM requirements dramatically. For example, 4GB RAM should be good for 2-3 OS' to come, or longer.

Proof is that base MBAs ship with 4. If Apple is selling machines that should last the next 5-6 years (or at least 3), then they don't want to intentionally alienate those who have bought the machines with only 4GB RAM.

 

yes of course, it's not that fast, but it does happen gradually. I had a 2006 MacBook pro. Original amount of ram (and it was never upgraded) was 1GB. That's good for tiger (which it shipped with) and leopard, but then snow leopard required 1GB. It was much slower with Snow Leopard.

An ideal amount of ram to have is twice the OS's minimum.

 

Future softwares will be more demanding because of a better hardware technology. It is sad but that is the way it is.

 

People say that you should always update to the latest version of their OS. This is mostly not true, IMO.

 

Mostly computers these days don't gradually decline over time as such, what they do is their apparent performance reduces in a series of steps, some large, some small.

Barring faults (such as fans clogging leading to a sudden 50% reduction because the computer has throttled itself to prevent damage), these steps are mostly because we ask the computer to do something additional (like a new program, or a new background service like loud syncing of some kind), or we change an old app for a new one and the developers have added features and functionality.

Apart from this your cpu will clock at the same speed when 10yrs old as when it was new, memory read access times just the same. Again unless it is starting to fail even an HDD will likely see the same read and write speeds - although we may give it more to read and write because of the changes above, new stuff to do and bigger old stuff.

One aspect that can lead to a gradual decline is your HDD filling up however that is easy to monitor and control and should be normal housekeeping.

 

Just to clarify, it's a SSD drive. I point this about HDD and SSD's have different lifespans and different problems.

Hard Disks usually slow down due to gradual degradation, fragmentation... and that's all I can think of.

Solid State Drives slow down to filling up more than HDD's do; there are limited read/writes, so the less space there is, the less space for fault tolerance there is. Once a 'cell' dies, another cell can take the data's place, but if you have very little, it's hard to find room for misplaced data. Fragmentation is not a problem for SSD's because the read time is so quick anyways.

Both disks suffer from filling up because there is less space for caching.

But HDD's are slower in the first place.

Anyways, he has a SSD, not an HDD.

 

Fair point but we should still view disk space as a consumable and he seemed to be primarily interested in component degradation which on the whole does not occur. Very few users will hit SSD lifetime limits in the life of the device so remaining space and its effects is likely the only effect he will suffer. Most if not all SSDs have additional blocks of cells to account for lifetime issues over and above their stated capacity AFAIK.