XLL Add-ins

Introduced in Excel 1995.
These can be browsed to from the Excel add-ins dialog box ?

link - learn.microsoft.com/en-us/office/client-developer/excel/excel-xll-sdk-api-function-reference 
link - learn.microsoft.com/en-us/office/client-developer/excel/programming-with-the-c-api-in-excel 
link - learn.microsoft.com/en-us/office/client-developer/excel/asynchronous-user-defined-functions 
link - learn.microsoft.com/en-us/office/client-developer/excel/developing-dlls 
link - learn.microsoft.com/en-us/office/client-developer/excel/creating-xlls 
link - learn.microsoft.com/en-us/office/client-developer/excel/how-to-access-dlls-in-excel 
link - learn.microsoft.com/en-us/office/client-developer/excel/calling-user-defined-functions-from-dlls 
link - learn.microsoft.com/en-us/office/client-developer/excel/multithreaded-recalculation-in-excel 

An XLL is only loaded once per session

Registry Keys

If a .xll file has been added to the registry with an OPENx
If the file cannot be found then a message will be displayed saying just that.
Clicking on the entry in the Excel Add-ins dialog box will then prompt you and allow you to remove it.

C++ /CLI

This is Microsoft's managed version of C++.

C Programming Language

Learning how to write XLLs using the SDK requires you to learn the C programming language.
The C programming language is a very low level language and is extremely complicated.
The C API has none of the higher-level rapid development features of VBA, COM, or .NET.
Memory management is low level and, therefore, puts greater responsibility on the developer.
If the memory management is not implemented correctly Excel will freeze / crash or the processes will not terminate.
Many Excel features are not exposed to the C API, like they are with VBA, COM or .NET Framework.

Excel 2007 - Multi Threaded Calculation

Excel 2007 introduced the concept of multi-threaded recalculation of workbooks.
The Excel calculation thread calls UDFs in a serial fashion, one after the other, waiting for each call to complete before proceeding down the calculation chain.
If a UDF takes 2 seconds to complete, calling the function 5 times on a worksheet could take 10 seconds to complete.
Multithreaded recalculation will not really help in this situation because it does not scale very well.
Multiple threads all have their own overhead, and therefore it is not practical to define as many threads as you have calls to the UDF.

Excel 2010 - Asynchronous Functions using the SDK

Excel 2010 introduced support for calling user-defined functions (UDF) asynchronously using the SDK.
This can improve performance by enabling several calculations to run at the same time.
Asynchronous functions still must be completed as part of Excel's normal calculation phase.
Asynchronous functions can be run concurrently, but Excel will still show as calculating until all asynchronous functions have returned.

XLLaddins.com

link - github.com/keithalewis/xll12 

XLW

XLW wraps the C API described in Microsoft Excel 97 Developer's Kit in a few classes that make API programming with Excel much, much easier

link - github.com/xlw/xlw 
link - xlw.github.io/ 

XLLPlus

link - planettechsolutions.com 
link - planatechsolutions.com/xllplus6-online/dotnet_requirements.htm 
link - as-ltd.co.uk/xllplus/default.htm