Performance is affected by a myriad of factors, many of which aren't visible by looking at the worksheet.

For example, XLS/XLSX support a special "shared formula" representation which tells Excel that a formula is structurally similar to other formulae. This is not always written, depending on how the file was generated and other factors. Without that, Excel will take an "inefficient" calculation approach. You can't figure it out from the UI or by inspecting the formula text -- you actually have to dig into the file to see it.