PHP and MySQL are capable of handling a high work load, but that has nothing to do with users uploading large files. Any user who is trying to upload large files with a slow dial-up connection is at the mercy of the connection bandwidth. The server-side language and database will not make much difference. It can't speed up the transfer.

Do I understand your question correctly? If not, please elaborate.

Agree with topdogger.

You may also want to check with your hosting company their server hardware performance, i.e. gigabit networ adaptor, gigabyte ram etc.

i agree with them both you will want to check with the hosting provider