SELECT 
  cscart_product_prices.product_id, 
  COALESCE(
    cscart_master_products_storefront_min_price.price, 
    MIN(
      IF(
        cscart_product_prices.percentage_discount = 0, 
        cscart_product_prices.price, 
        cscart_product_prices.price - (
          cscart_product_prices.price * cscart_product_prices.percentage_discount
        )/ 100
      )
    )
  ) AS price 
FROM 
  cscart_product_prices 
  LEFT JOIN cscart_master_products_storefront_min_price ON cscart_master_products_storefront_min_price.product_id = cscart_product_prices.product_id 
  AND cscart_master_products_storefront_min_price.storefront_id = 1 
WHERE 
  cscart_product_prices.product_id IN (
    70149, 69350, 69997, 65553, 65832, 65096, 
    67762, 68110, 67730, 68008, 68009, 
    68052, 68065, 65122, 68039, 68071, 
    68085, 68086, 68101, 64976, 67769, 
    68057, 68058, 69972, 19741, 19744, 
    19748, 27363, 27368, 27370, 68038, 
    68089, 68099, 69999, 19742, 19743, 
    19745, 19746, 19747, 19749, 19750, 
    19751, 19752, 19753, 19754, 19755, 
    27337, 27339
  ) 
  AND cscart_product_prices.lower_limit = 1 
  AND cscart_product_prices.usergroup_id IN (0, 1) 
GROUP BY 
  cscart_product_prices.product_id

Query time 0.00383

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9.91"
    },
    "grouping_operation": {
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_master_products_storefront_min_price",
            "access_type": "system",
            "possible_keys": [
              "PRIMARY"
            ],
            "rows_examined_per_scan": 0,
            "rows_produced_per_join": 1,
            "filtered": "0.00",
            "const_row_not_found": true,
            "cost_info": {
              "read_cost": "0.00",
              "eval_cost": "0.10",
              "prefix_cost": "0.00",
              "data_read_per_join": "16"
            },
            "used_columns": [
              "storefront_id",
              "product_id",
              "price"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_product_prices",
            "access_type": "range",
            "possible_keys": [
              "usergroup",
              "product_id",
              "lower_limit",
              "usergroup_id",
              "idx_product_prices_usergroup_limit",
              "idx_prices_discount_lookup",
              "idx_prices_min_calc",
              "idx_prices_usergroup_lookup",
              "idx_prices_usergroup_product"
            ],
            "key": "idx_prices_discount_lookup",
            "used_key_parts": [
              "product_id",
              "lower_limit"
            ],
            "key_length": "6",
            "rows_examined_per_scan": 48,
            "rows_produced_per_join": 9,
            "filtered": "20.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "8.95",
              "eval_cost": "0.96",
              "prefix_cost": "9.91",
              "data_read_per_join": "230"
            },
            "used_columns": [
              "product_id",
              "price",
              "percentage_discount",
              "lower_limit",
              "usergroup_id"
            ],
            "attached_condition": "((`cscart_db`.`cscart_product_prices`.`lower_limit` = 1) and (`cscart_db`.`cscart_product_prices`.`product_id` in (70149,69350,69997,65553,65832,65096,67762,68110,67730,68008,68009,68052,68065,65122,68039,68071,68085,68086,68101,64976,67769,68057,68058,69972,19741,19744,19748,27363,27368,27370,68038,68089,68099,69999,19742,19743,19745,19746,19747,19749,19750,19751,19752,19753,19754,19755,27337,27339)) and (`cscart_db`.`cscart_product_prices`.`usergroup_id` in (0,1)))"
          }
        }
      ]
    }
  }
}