Fine tune Excel VBA Macro for multi-thread processing

General information for the business: Computing
Kind of development: Customization of existing program
Description of requirements/functionality: We have a spreadsheet which has been written by a non-professional coder using Macros/VBA within Excel. The spreadsheet is compiled of 2 x Excel files using handful of sheets.

Whilst the spreadsheet works for the purpose in which is was intended, it does not operate well on the Windows Server environment very well. It looks as though the spreadsheet has not been coded with multi-thread processing in mind.

For example when the spreadsheet runs on a computer with few processing cores but large cores, i.e. 3Ghz it executes functions with no problem. However when the spreadsheet is run on server with multiple cores but running at a less speed 2.2 Ghz, it often produces the error message "Excel ran out of resources while attempting to calculate one or more formulas. As a result, these formulas cannot be evaluated".

Also the spreadsheet even at idle (when the user has it open but is not executing functions) consumes a lot of CPU - 5% per instance of overall server CPU per user. When this is times 8 users it becomes a problem!

So ultimately we need someone to tune the file to use less CPU as a whole and also make it more compatible for our server environment (which runs multiple CPU cores).

Thanks
Specific technologies required: Excel VBA Visual Basic Macros
OS requirements: Windows
Extra notes: