if i was you, i will try to add somekind of index to your MySQL database for this fields:

when this is done i will changes your SQL to a kind of this SQL

SELECT DISTINCT 
            cb.customers_id, 
            cb.customers_basket_date_added, 
            c.customers_firstname, 
            c.customers_lastname, 
            c.customers_email_address, 
            c.customers_telephone, 
            sc.datemodified AS last_contacted
            
            FROM 
                customers_basket cb INNER JOIN customers c ON c.customers_id = cb.customers_id
                LEFT JOIN scart sc ON cb.customers_id = sc.customers_id
                
            WHERE 
                ( cb.customers_basket_date_added BETWEEN '{start-date}' AND '{end-date}' ) AND
                NOT EXISTS (
                    SELECT  
                        sc.customers_id 
                    
                    FROM 
                        scart sc 
                    
                    WHERE
                        sc.datemodified > '{date-modified}' AND
                        sc.{customerId-field} = cb.customers_id )
                        
            ORDER BY 
                cb.customers_basket_date_added DESC

i have change your NOT IN function and changes it to NOT EXISTS, and change a lot more.

why are you using distinct and not group by function?

when this SQL perfome fast, we can go to next step the SQL inside your loop, is it posible you can make you SQL dump so i can download it to help you more? i think you have a big mistake in your 3 SQL, maby its can be done width only 1 SQL select and not 1 select * 2 select each loop run, this mean if you got 90 rows out you make 181 selects, its take a big perfome from you MySQL and i pretty sure you can do it on only one select, :)

If I were you, I would try to add some sort of index to your MySQL database for this fields:

when this is done I would make changes your SQL, something similar to this:

SELECT DISTINCT 
    cb.customers_id, 
    cb.customers_basket_date_added, 
    c.customers_firstname, 
    c.customers_lastname, 
    c.customers_email_address, 
    c.customers_telephone, 
    sc.datemodified AS last_contacted
FROM 
    customers_basket cb  
    INNER JOIN customers c ON c.customers_id = cb.customers_id
    LEFT JOIN scart sc ON cb.customers_id = sc.customers_id
WHERE 
    ( cb.customers_basket_date_added BETWEEN '{start-date}' AND '{end-date}' ) AND
    NOT EXISTS (
                SELECT  
                    sc.customers_id 
                FROM 
                    scart sc 
                WHERE
                    sc.datemodified > '{date-modified}' AND
                    sc.{customerId-field} = cb.customers_id )
                ORDER BY 
                    cb.customers_basket_date_added DESC

I changed your NOT IN function to NOT EXISTS, and changed a lot more.

Why are you using distinct and not group by function?

when this SQL performs fast we can go to next step, the SQL inside your loop.

Is it posible to make a SQL dump so that I can download it and help you out more?

I think you have a big mistake in your 3rd block of SQL, maybe it can be done with only 1 SQL select and not 1 select * 2 select each loop run, this mean if you got 90 rows out you make 181 selects, I am sure that you can do it with less select statements being run against the SQL Server.