There is a bug in the current version of ArcGIS that causes very slow performance when it is used on corporate networks such as the one at Sussex. The makers of the software are aware of this issue but it may be some time before it is resolved.
In the meantime, you can speed up the performance of ArcGIS considerably by working on files on the local C:drive of the computer you are using and then copying them back to your N:drive when you have finished.
For example, if you are working on a new map:
After this you can work on your map as normal and then, when you've finished working on it:
The next time you need to work on the map, simply copy the files back to C:\Temp, then open ArcGIS - you will then be able to open your map to continue working.
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