First of all,,,, let me ask a question ,,,, Is there any other type of files e.g. PDFs , MsWord etc. shared on the same Network Drive where the Shared XLS reside ????? if there is any please check if they have the same slow performance problem or not …..
Secondly ,, sharing a file on the a server and allowing users to access it and modify things on it put some load on the file , server and the Network performance . So, I recommend if you can find another way to do this job like putting in on a site of the intranet of your company this will be much better .