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 (
    12656, 12658, 12660, 12661, 12662, 12664
  )

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.0215057,
    "nested_loop": [
      {
        "table": {
          "table_name": "pfv",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "loops": 1,
          "rows": 6,
          "cost": 0.01122386,
          "filtered": 100,
          "attached_condition": "pfv.variant_id in (12656,12658,12660,12661,12662,12664)"
        }
      },
      {
        "table": {
          "table_name": "pfvd",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["variant_id", "lang_code"],
          "ref": ["u508912950_dev_2025.pfv.variant_id", "const"],
          "loops": 6,
          "rows": 1,
          "cost": 0.01028184,
          "filtered": 100,
          "attached_condition": "pfvd.lang_code = 'en'"
        }
      }
    ]
  }
}