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 (
    12532, 13061, 12535, 12538, 12539, 12528, 
    13062, 13063, 12530, 12533, 12537, 
    12531, 12536
  )

Query time 0.00019

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.04157671,
    "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": 13,
          "cost": 0.02339539,
          "filtered": 100,
          "attached_condition": "pfv.variant_id in (12532,13061,12535,12538,12539,12528,13062,13063,12530,12533,12537,12531,12536)"
        }
      },
      {
        "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": 13,
          "rows": 1,
          "cost": 0.01818132,
          "filtered": 100,
          "attached_condition": "pfvd.lang_code = 'en'"
        }
      }
    ]
  }
}