SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    187, 318, 319, 320, 321, 322, 323, 324, 
    325, 326, 327, 328, 329, 330, 331, 332, 
    333, 334
  ) 
  AND feature_id IN (550, 549, 548) 
  AND lang_code = 'en'

Query time 0.00128

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "26.05"
    },
    "table": {
      "table_name": "cscart_product_features_values",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "fl",
        "lang_code",
        "product_id",
        "fpl",
        "idx_product_feature_variant_id"
      ],
      "key": "idx_product_feature_variant_id",
      "used_key_parts": [
        "product_id",
        "feature_id",
        "lang_code"
      ],
      "key_length": "12",
      "rows_examined_per_scan": 57,
      "rows_produced_per_join": 57,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "14.65",
        "eval_cost": "11.40",
        "prefix_cost": "26.05",
        "data_read_per_join": "44K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`atulecarter_atul_demo2`.`cscart_product_features_values`.`product_id` in (187,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334)) and (`atulecarter_atul_demo2`.`cscart_product_features_values`.`feature_id` in (550,549,548)) and (`atulecarter_atul_demo2`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id feature_id variant_id
187 548 1194
187 549 1199
187 550 1202
318 548 1193
318 549 1199
318 550 1202
319 548 1197
319 549 1199
319 550 1202
320 548 1194
320 549 1200
320 550 1202
321 548 1195
321 549 1200
321 550 1202
322 548 1196
322 549 1200
322 550 1202
323 548 1193
323 549 1201
323 550 1202
324 548 1195
324 549 1201
324 550 1202
325 548 1196
325 549 1201
325 550 1202
326 548 1197
326 549 1199
326 550 1203
327 548 1195
327 549 1199
327 550 1203
328 548 1196
328 549 1199
328 550 1203
329 548 1194
329 549 1200
329 550 1203
330 548 1195
330 549 1200
330 550 1203
331 548 1196
331 549 1200
331 550 1203
332 548 1194
332 549 1201
332 550 1203
333 548 1195
333 549 1201
333 550 1203
334 548 1196
334 549 1201
334 550 1203