SELECT 
  cscart_vendor_following.user_id, 
  followed_on, 
  firstname, 
  lastname, 
  email 
FROM 
  cscart_vendor_following 
  LEFT JOIN cscart_users ON cscart_users.user_id = cscart_vendor_following.user_id 
WHERE 
  cscart_vendor_following.vendor_id = 5 
ORDER BY 
  followed_on desc

Query time 0.00044

JSON explain

{
  "query_block": {
    "select_id": 1,
    "read_sorted_file": {
      "filesort": {
        "sort_key": "cscart_vendor_following.followed_on desc",
        "table": {
          "table_name": "cscart_vendor_following",
          "access_type": "ALL",
          "rows": 5,
          "filtered": 100,
          "attached_condition": "cscart_vendor_following.vendor_id = 5"
        }
      }
    },
    "table": {
      "table_name": "cscart_users",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["user_id"],
      "ref": ["devmotorsbay.cscart_vendor_following.user_id"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(cscart_users.user_id = cscart_vendor_following.user_id)"
    }
  }
}