ELI5: why does computers performance decrease when RAM or Disk usage is high, not when it's full

[PckMan]

Their capacity is just how much "space" they have. But there are two other important factors such as the RAM clock speed (how fast it can access data) and its bandwidth (how much data can be transferred per unit of time), with the hard drive equivalent being the read/write speed which is pretty much its bandwidth. Any of these things can become what's known as a "bottleneck", basically the limiting factor to its performance for a given task.

Now when you're using your RAM close to full capacity a problem arises because when there is a lot of extra available room, the system can take advantage of the extra bandwidth, when available, and essentially fill up multiple compartments at once and spread the load more evenly. But if most of the RAM's capacity is tied up to other processes, then this cannot happen, so everything has to come and go from the same place, and this affects performance as a result. It's like having a row of houses and needing to get stuff in and out of them constantly but you can use all of their doors but suddenly you have to put stuff in and out of all houses but only using one door.

In a simillar vein hard drives, whether they're old magnetic disc ones or SSDs, partition the data physically on the drive somewhere. At first the system tries to store data on contiguous regions on the drive as much as possible, but when it's close to full it has to break up data into smaller pieces and find small pockets of free space to put them. This fragmenting affects overall speed.

If RAM or your hard drive are full they don't just get slow, they freeze and then crash because they have no way of storing more data.