SQL window functions provide extremely powerful and useful features that let you work with observations more easily. Different types explained here.

When I switched the compatibility mode to 150 (using SQL Server 2019), the same query took 900 milliseconds and the execution plan showed all of the detail of the function's execution.

What Is Tsql Function? Users-defined functions are Transact-SQL or CLRS (common language runtime) routines that take parameters, perform actions, such as complex calculations, and return the results ...