Why parameter for one query returning the same # of results as a different parameter, is extremely disproportionately slower
Look at the execution plans.

SQL Server will build different execution plans based on the different number of rows that it expects to return. (Even though the ACTUAL number of rows that may be returned could be the same, the ACTUALS could be different.)

To get the actual execution plans in SSMS, click Query, Include Actual Plan. Then you can compare the different plans to see if they have different shapes.

If you're using SQL Server Management Studio 2016, you can use the Compare Showplan feature to make that easier. (It works even for querying 2008 boxes.)

If you can share the execution plans publicly, use PasteThePlan.com to upload the two different execution plans, and then folks can point out what's different between them. (Be aware that this tool is completely public - everyone will see the full text of your query, and your parameters.)