SELECT 
  pfv.variant_id, 
  pfv.position, 
  pfvd.variant 
FROM 
  cscart_product_feature_variants AS pfv 
  INNER JOIN cscart_product_feature_variant_descriptions AS pfvd ON pfv.variant_id = pfvd.variant_id 
  AND pfvd.lang_code = 'en' 
WHERE 
  pfv.variant_id IN (
    1935, 1949, 2131, 1836, 1660, 1954, 2378, 
    2143, 2130, 2132, 2129, 1803, 2381, 
    1712, 2424, 2415, 2423, 2173, 2426
  )

Query time 0.00057

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "19.96"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "pfv",
          "access_type": "range",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id"
          ],
          "key_length": "3",
          "rows_examined_per_scan": 19,
          "rows_produced_per_join": 19,
          "filtered": "100.00",
          "index_condition": "(`test2_gaseus`.`pfv`.`variant_id` in (1935,1949,2131,1836,1660,1954,2378,2143,2130,2132,2129,1803,2381,1712,2424,2415,2423,2173,2426))",
          "cost_info": {
            "read_cost": "11.41",
            "eval_cost": "1.90",
            "prefix_cost": "13.31",
            "data_read_per_join": "21K"
          },
          "used_columns": [
            "variant_id",
            "position"
          ]
        }
      },
      {
        "table": {
          "table_name": "pfvd",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "test2_gaseus.pfv.variant_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 19,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "4.75",
            "eval_cost": "1.90",
            "prefix_cost": "19.96",
            "data_read_per_join": "59K"
          },
          "used_columns": [
            "variant_id",
            "variant",
            "lang_code"
          ]
        }
      }
    ]
  }
}