SELECT 
  * 
FROM 
  cscart_product_feature_variants 
WHERE 
  variant_id IN (
    1934, 1862, 1855, 1939, 1874, 3101, 3102, 
    3087, 3090, 3093, 3094, 1856, 1859, 
    1870, 0, 229, 231, 723, 511, 658
  )

Query time 0.00063

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_feature_variants",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "rows": 20,
          "filtered": 100,
          "index_condition": "cscart_product_feature_variants.variant_id in (1934,1862,1855,1939,1874,3101,3102,3087,3090,3093,3094,1856,1859,1870,0,229,231,723,511,658)"
        }
      }
    ]
  }
}