Did you investigate what the real bottleneck is? This is all about upload speed, and the most obvious cause is that the clients have not enough bandwidth. Even if they use a fast ADSL, they could still have low upload speed (the “A” in ADSL stands for “Asymmetric”, i.e. fast download, but slow upload).
For example, with a typical (for Europe) upload speed of 512kb/s, to upload a 100MB file it will take at least 26 minutes, assuming full speed (which is not the usual case)…
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solved Uploading files are very slow in my PHp project