Select substring faster performance than select column

I am working on an error in my application and found something that I don't understand.

One of my columns has the type nvarchar(max) and contains strings with a length of 65k chars.

Selecting the column in my query causes a performance issue,

SELECT Id
       ,Description
       ,DescriptionForUser
FROM MyTable

The query takes 15 sec to execute, but when using substring(colname,1,100000)

SELECT Id
       ,Description
       ,SUBSTRING(DescriptionForUser,1,100000)
FROM MyTable

The query execution takes under a second.

What causes this performance change?