SELECT 
  * 
FROM 
  vi_product_feature_variants 
WHERE 
  variant_id IN (91, 39, 65, 45)

Query time 0.00009

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "vi_product_feature_variants",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "rows": 4,
          "filtered": 100,
          "index_condition": "vi_product_feature_variants.variant_id in (91,39,65,45)"
        }
      }
    ]
  }
}