In CSOM FieldUserValue.LookupValue returns the display name of the user. Alternatively, you can enter user details from the & # 39; user information list & # 39; query by passing the user ID. However, you must do this with ClientContext.ExecuteQuery (), which you want to avoid.
If you want, use REST API then you can use below
_api / web / lists / getbytitle (# listname #) / items (id)? $ select = YourUserField / Name & $ expand = YourUserField / Id
Above the endpoint URL is & # 39; Name & # 39; returned, which is similar to the login name in sharePoint online. REST is also a simple and fast way to retrieve data.