SELECT 
  a.weight_modifier as modifier, 
  a.weight_modifier_type as modifier_type, 
  a.variant_id 
FROM 
  cscart_product_option_variants a 
WHERE 
  1 
  AND a.variant_id IN (314, 318, 502)

Query time 0.00011

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "a",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["variant_id"],
      "rows": 3,
      "filtered": 100,
      "attached_condition": "a.variant_id in (314,318,502)"
    }
  }
}