SELECT 
  object_id 
FROM 
  cscart_images_links 
WHERE 
  object_type = 'abt__yt_mwi__icon'

Query time 0.00020

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_images_links",
          "access_type": "index",
          "key": "object_id",
          "key_length": "81",
          "used_key_parts": ["object_id", "object_type", "type"],
          "rows": 419,
          "filtered": 100,
          "attached_condition": "cscart_images_links.object_type = 'abt__yt_mwi__icon'",
          "using_index": true
        }
      }
    ]
  }
}