Why my simple searching query is too slow?

I don’t know why searching in my “cities” table is so slow.
My query looking for a table “cities” located about 25km from the city.
I use this simple query and the database takes almost 20 seconds to return results.

SELECT city_destination,distance FROM cities
     WHERE city_start='Wien'
       AND distance <= 25
     ORDER BY distance ASC

Table engine is InnoDB. The table has approx. 7 millions of rows:

+--------------------+-------------+------+-----+---------+----------------+
| Field              | Type        | Null | Key | Default | Extra          |
+--------------------+-------------+------+-----+---------+----------------+
| id                 | int(11)     | NO   | PRI | NULL    | auto_increment |
| id_of_start        | int(11)     | NO   |     | NULL    |                |
| id_of_destination  | int(11)     | NO   |     | NULL    |                |
| city_start         | text        | NO   |     | NULL    |                |
| city_destination   | text        | NO   |     | NULL    |                |
| distance           | double      | NO   |     | NULL    |                |
+--------------------+-------------+------+-----+---------+----------------+

Can anyone advise me how to optimize a database or query?

Just guessing what’s happening as you’ve not provided a query plan or the indexes you’ve created.

Assuming you expect this query to return relatively few rows.

create index my_new_index on cities (city_start, distance );

Order of the columns in the index is important, you only have an equality filter on city_start so this should come before distance otherwise it can’t be used as part of the index selectivity (see https://ctandrewsayer.wordpress.com/2017/03/24/the-golden-rule-of-indexing/ for more info)