When copying or moving files (in Windows for this example) it shows you the speed at which the transfer is currently happening. However this is very rarely a constant rate and can vary wildly from second to second. What factors are involved in affecting this speed?
In: Technology
How your computer is being taxed by running other programs. Your computer can only handle so many tasks at once, so when you have multiple programs running it sometimes has to juggle all those tasks, causing slow downs in your copying.
It also has to do with file size, if you’re copying a bunch of photos for example, ones that have smaller files sizes will copy quicker than those that are larger, but if you’re just looking at a “photo X out of 100” progress bar it just seems like some are randomly causing slowdowns.
When your computer copies a file, there are some steps that take the same amount of time regardless of the size of the file, and there is a step that takes longer when the file is longer– namely, copying the actual data. Because of this, there is a difference in time between, say, copying 1 file that is 1GB, and copying 10000 files that add up to 1GB.
The larger the number of files, the longer it will take to copy that amount of data.
Windows’s file copy progress bar displays things as a percentage of the amount of data copied. So during the copy process, if it gets a bunch of small files in a row, it will slow down. If it gets mostly large files in a row, it will speed up.
I once gave the example of when you have a few large items it is easier to allocate a place to put them compared to many small items.
If I point at a brick and ask you to place it on a shelf that is easy and fairly quick, however if I ask you to move a bricks worth of dust to the same shelf it maybe a bit more difficult. If we put the brick with of dust into a container (zip file) or ziplock bag it would become easier again.
Adding to the other answers that people have mentioned, I want to point out that the drive itself lies to the computer about where it copies.
Often what will happen is that the drive itself has a small write cache, a place where it will write files before permenantly writing them to the disk. Writing to this cache is often a lot faster than writing to disk. This saves computer resources since once that is done, the drive itself handles the rest of doing the final copy from cache to the drive, leaving the cpu to do whatever it wants. This is convenient especially because large copies are pretty rare from the computers point of view, most programs will only write small files at a time and this caching helps free resources.
If the file is too big, it will often fill up this cache, at which point the disk complains to the OS that it is copying too fast and asks it to wait while it clears the cache to disk to receive more files. This is why the first few megabytes of a file copy fast but afterwards it slows down a lot.
– how many other programs are accessing the disk at the time
– the mechanics of the storage device itself. for example if it’s a hard drive then it may have to rotate the platters to write some of the data and that will introduce delays.
– the size of the files also matters. larger files can be written in contiguous blocks so they tend to be faster to write.
– buffers and caches that are in place. for example you may have a cache in main memory. writing to that is faster but it has limited space. then there’s an on-drive buffer in most drives. when you exceed the space you will face slowdowns.
– the filesystem used will also affect read/write speeds.
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