The problem is either with the file (formulas, addins, link refs, formula names, etc), network connection, your pc or the server.
After we answer these questions we'll most certainly get to the bottom of it.
1. Right Click Start, click Run, type Excel /s and hit Enter. This starts Excel with no external links, addins, etc. Browse to the file and open it and monitor the speed. Is it faster?
2. Run a speed test and anything lower than 100mbps is insufficient for truly large excel files to open quickly: https://www.google.com/search?q=speed+test&rlz=1C1GCEO_enUS978US978&oq=speed+test&aqs=chrome..69i57j0i131i433i512j0i433i512j0i10i433i512j0i433i512j0i131i433i512j0i433i512l2j0i512.3286j0j7&sourceid=chrome&ie=UTF-8
3. Does it run faster for anyone else?
4. Does it run much faster when it is downloaded to your pc or if it's copied to Box ?
The usual problem we face with excel files from Deloitte, GS, MS, etc. is that they sometimes fail to remove external references to their own systems. Check the Data tab, and in the Queries & Connections section click Edit links* and disable any links that we haven't created. If the file uses Macabacus then we'll likely need to use name scrubber or the file will continue to be nearly unusable. If they have a lot of named formula references on there that could lead to formatting bloat the an artificially large file size compared to its visible contents. To see if that's the case, click the Formulas tab, in the Defined Names group, click Name Manager**.
*
**
If the problem is with named formula references Macabacus provides a tool (Name Scrubber) to clean these references up on the sheet and to eliminate this 'calculation fatigue'. You may find that the file size drops by up to 80% after Name scrubber is run. More on name scrubber here. More on Excel formula names here.
Warning: The Name Scrubber is a Macabacus function. There are often tens of thousands of names it needs to go clean up so caution the user to schedule this activity. It can take a while, and since version 9.0.9 cleaning names can now be canceled by the user.