SELECT 
  `data` 
FROM 
  cscart_storage_data 
WHERE 
  `data_key` = 'dev_mode'

Query time 0.00018

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_storage_data",
      "access_type": "const",
      "possible_keys": [
        "PRIMARY"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "data_key"
      ],
      "key_length": "767",
      "ref": [
        "const"
      ],
      "rows": 1,
      "filtered": 100
    }
  }
}