MySQL is pretty slow after 50-100k rows whats happen?

I have been trying now many times, and I tried to find a way to make sense of and find out my problem. But the problem is still performance issues. I tested it on a developer server ( 2 cores, 4gb ram ) but I still got issues and problems, here is the stored procedure I have typed.

DROP PROCEDURE IF EXISTS insertDistributorProduct;
    
    DELIMITER $$
    
    CREATE PROCEDURE insertDistributorProduct(
    	distributorUUID VARCHAR(36),
    	distributorTitle VARCHAR(256),
    	distributorDescription TEXT,
    	distributorSKU VARCHAR(128),
    	manufacturer VARCHAR(64),
    	manufacturerSKU VARCHAR(128),
    	productEan BIGINT(20),
    	productCostPrice INT(11),
    	productStock INT(11),
    	productStockExpected DATE
    )
    BEGIN
    	DECLARE _manufacturerUUID VARCHAR(36) DEFAULT NULL;
    	DECLARE _distributorProductUUID VARCHAR(36) DEFAULT NULL;
    	
    	SET _manufacturerUUID = ( SELECT m.manufacturerUUID FROM manufacturer m
                WHERE LOWER(m.manufacturerTitle) = LOWER(manufacturer) LIMIT 1 );
    
    	-- If manufacturer not exists insert it
    	IF ( _manufacturerUUID IS NULL ) THEN
    		
    		SET _manufacturerUUID = ( SELECT ma.manufacturerUUID FROM manufacturerAlias ma
                    WHERE LOWER(ma.aliasTitle) = LOWER(manufacturer) LIMIT 1 );
    		
    		-- _manufacturerUUID not found in manufacturer and _manufacturerAlias so insert
    		IF ( _manufacturerUUID IS NULL ) THEN
    		
    			SET _manufacturerUUID = UUID();
    			
    			INSERT INTO manufacturer(manufacturerUUID,manufacturerTitle,added)
    			VALUES(_manufacturerUUID,manufacturer,NOW());
    			
    		END IF;
    			
    	END IF;
    		
    	SET _distributorProductUUID = ( SELECT dp.distributorProductUUID FROM distributorProduct dp
                WHERE dp.distributorUUID = distributorUUID
                  AND LOWER(dp.distributorSKU) = LOWER(distributorSKU) );
    	
    	-- Update distributorProduct if its all ready found
    	IF ( _distributorProductUUID IS NOT NULL ) THEN
    
    		UPDATE 
    			distributorProduct dp
    		
    		SET 
    			dp.distributorTitle = distributorTitle,
    			dp.distributorDescription = distributorDescription,
    			dp.manufacturerUUID = _manufacturerUUID,
    			dp.manufacturerSKU = manufacturerSKU,
    			dp.productEan = productEan,
    			dp.productCostPrice = productCostPrice,
    			dp.productStock = productStock,
    			dp.productStockExpected = productStockExpected,
    			dp.updated = NOW(),
    			dp.activated = 1
    			
    		WHERE
    			dp.distributorProductUUID = _distributorProductUUID
    		
    		LIMIT
    			1;
    	
    	-- Insert new product to distributorProduct if not exists.
    	ELSE
    	
    		INSERT INTO distributorProduct (distributorProductUUID,distributorUUID,manufacturerUUID,distributorTitle,distributorDescription,distributorSKU,manufacturerSKU,productEan,productCostPrice,productStock,productStockExpected,added)
    		VALUES(UUID(),distributorUUID,_manufacturerUUID,distributorTitle,distributorDescription,distributorSKU,manufacturerSKU,productEan,productCostPrice,productStock,productStockExpected,NOW());
    	
    	END IF;
    	
    END
    $$

My problem is that every time I have over 50-100k rows, this script takes longer and longer over time, and I have to try to look up my index but still nothing happens. How can I debug and improve the performance of my database? Every day I need to run this producer like 1-1,4milion times between 4-5 hours max, it updates all our products but right now it's useless. The first time 60k rows take 15min, and after that 60k more ( total 120k ) it takes approx 30-45min to run, and I still don't know why it happened. What did I do wrong? Everything I tested here is running in a virtual box software, I don't have to test this on real hardware.