Slow Queries SQL Server Full-Text

I have created a catalog for Full-Text searches in SQL Server:

CREATE TABLE [dbo].[full_text](
[client_id] [int] NOT NULL,
[file_id] [bigint] NOT NULL,
[page_number] [int] NULL,
[contents] [nvarchar](max) NULL,
[ID] [int] IDENTITY(1,1) NOT NULL,
CONSTRAINT [PK_full_text] PRIMARY KEY CLUSTERED 
(
[ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF,        ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

GO

create fulltext catalog dd_full_text;
GO

create fulltext index on full_text(contents)
key index PK_full_text on dd_full_text with change_tracking auto
GO

I use the following query as test:

SELECT *
FROM full_text
WHERE CONTAINS(contents, '"ban*" AND NOT "sab*"');
GO

The first consultations are relatively fast but as the minutes pass they slow down as shown in the following table:

Response Time ms

During the tests I have not inserted new rows, so I do not understand why each time the queries take longer to return the results.