Is it permissible to use a wireless device to search for internet subscriptions and use them without payment or permission, knowing that this does not harm their owners but rather slows down their internet speed?

If the use of the device slows down the internet for other subscribers, then it is not permissible except with their permission, because it causes harm to them. This is based on the saying of the Prophet, peace and blessings be upon him: "Whoever harms others, Allah will harm him; and whoever causes hardship to others, Allah will cause hardship to him."