Sounds like a fairly specific thing. How many users do you forsee simultaneously uploading? I doubt it'll overlap more than two at a time very frequently.
Let's say your site is really popular... let's say.... 720 uploads per day. That's 21,600 in a month... 259,200 in a year! Congrats ;)
That's an average of 30 per hour. Let's say that they all take 1 minute. Then, there's an average of 1 every two minutes in the hour, with half of that time used for processing, the rest resting.
In other words, the serving is working half the time.
And, sure, it would overlap, but I doubt you'd get more than 3-4 at a time very often. 2 at a time? Sure. 3? sometimes. 4? rarely. 10? hardly ever. more? maaaaybe once in a while.
And, really, many servers have a 30 second limit on the PHP script processing, so that would be even less time busy, assuming all of the processing were under 30 seconds (or you have something bigger to deal with).
In the end, this means that, sure, once in a while, you'll come across a time when multiple people are uploading. They'll get a slight delay. ...so?
I've had slight delays on many pro sites (my connection? Theirs? both?), but it's to be expected. Once in a while something loads slowly. The end. And it would be back up full speed soon enough.
