SELECT 
  cscart_product_prices.product_id, 
  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 
WHERE 
  cscart_product_prices.product_id IN (
    19860, 19820, 19863, 19865, 19866, 20154, 
    19867, 19868, 19869, 19870, 19871, 
    19872, 19873, 19875, 19874, 19876, 
    20087, 19877, 19878, 19879, 19880, 
    19881, 19882, 19883, 19884, 19885, 
    19886, 19888, 19889, 19890, 19891, 
    19892, 19893, 19894, 19895, 19896, 
    19897, 19898, 19899, 19900, 19901, 
    19902, 19903, 19904, 19905, 19925, 
    19906, 19907, 19908, 19909, 19910, 
    19911, 20576, 20088, 20089, 20091, 
    20092, 20093, 20094, 18661, 18662, 
    19131, 18664, 20724, 20725, 20728, 
    20955, 20956, 20957, 20958, 20959, 
    22277, 20971, 21078, 21102, 20972, 
    20973, 20974, 21103, 21923, 21924, 
    21925, 21926, 20065, 19887, 21749, 
    20723, 20726, 20727
  ) 
  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 0.01319

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "cscart_product_prices.product_id",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "cscart_product_prices",
              "access_type": "ref",
              "possible_keys": [
                "usergroup",
                "product_id",
                "lower_limit",
                "usergroup_id"
              ],
              "key": "lower_limit",
              "key_length": "3",
              "used_key_parts": ["lower_limit"],
              "ref": ["const"],
              "rows": 11706,
              "filtered": 100,
              "attached_condition": "cscart_product_prices.lower_limit <=> 1 and cscart_product_prices.product_id in (19860,19820,19863,19865,19866,20154,19867,19868,19869,19870,19871,19872,19873,19875,19874,19876,20087,19877,19878,19879,19880,19881,19882,19883,19884,19885,19886,19888,19889,19890,19891,19892,19893,19894,19895,19896,19897,19898,19899,19900,19901,19902,19903,19904,19905,19925,19906,19907,19908,19909,19910,19911,20576,20088,20089,20091,20092,20093,20094,18661,18662,19131,18664,20724,20725,20728,20955,20956,20957,20958,20959,22277,20971,21078,21102,20972,20973,20974,21103,21923,21924,21925,21926,20065,19887,21749,20723,20726,20727) and cscart_product_prices.usergroup_id in (0,1)"
            }
          }
        ]
      }
    }
  }
}