Does using an unsecured wireless internet network—while ensuring the owner is not harmed by slow speed—fall under the meaning of the Hadith: "A neighbor should not prevent his neighbor from inserting a piece of wood into his wall"?

It is not permissible to use private networks belonging to their owners without their permission, even if they are unsecured, because their owners pay money for their services. It is not permissible to use them unless their owners explicitly or implicitly permit it, based on the Prophet's (peace be upon him) saying: "The wealth of a Muslim person is not lawful [to take] except with his willing consent." As for networks provided by the state in public places for people to benefit from, there is no harm in accessing them.