You are utilizing Google BigQuery as your data warehouse solution. Users have raised concerns about the performance of a particular query which persists in running sluggishly, irrespective of the time of execution. The query in question is:

SELECT country, state, city FROM [myproject:mydataset.mytable] GROUP BY country

Upon examining the query plan within the Read section of Stage:1, you observe certain details that could be indicative of the underlying issue. What is the most probable reason for the delay in the execution of this query?