The scenario:
I need to copy 1 named range from the template 50,000 (or even more) times into the resulting file

When setting the cell value, the IsInferiorMergedCell() method is called, which significantly slows down the speed of operation with a large number of merged ranges. Although if I manually make a merge, I know in advance whether I need to set a value in this cell or not

Initially, I thought about making a public method SetValue(XLCellValue value, bool checkMergedRanges), which will avoid this check, but the problem is that the setter FormulaA1, which is called in the SetValue() method, also contains the IsInferiorMergedCell() call.

So I decided to just add the IsInferiorMergedCellUserDefined property, which will bypass this check. I admit that the solution is not the best, but it suits me. I wanted to highlight the problem more, maybe someone has a better solution right away

Code for reproduce: https://gist.github.com/Arteeck/32de98b54ec85cc9ae923766b73e7562

Template: template.xlsx

Sampling without IsInferiorMergedCellUserDefined:

Sampling with IsInferiorMergedCellUserDefined: