1) Who cares how long it takes to create a database? How often is that done, and how many of THOSE times does it need to complete 15 seconds faster?? 😎

2) You should always use 64K NTFS cluster size for volumes that are used for SQL Server data/log files to my knowledge

3) The CREATE could be slower than the alter because you have a tlog that is being zeroed out in the CREATE but the ALTER doesn't affect the tlog so Instant File Initialization allows the 10GB data file to be created in a flash. Also the CREATE has to make lots of new structures and also copy over everything from MODEL. Just some guesses there.