Thanks to let us know it.
The reason of that is when the sample build the new mbtils, it forget build index.
You can add this code into the logic when create new mbtiles file, or you can just run it once to build index for your existing mbtiles and it should also works.
MbtilesDatabase sourceDB = MbtilesDatabase.OpenDatabase(@"D:\Illinois3.mbtiles");
string querySql = "CREATE INDEX tile_id_index ON images(tile_id)";
Here is my test result, it's fast after build index.