These limits are related to entropy, space, energy, etc., so if you further assume the universe is finite (or perhaps equivalently, that the malicious agent can only access a finite portion of the universe due to e.g. speed-of-light limits), then there is an upon bound of information possible, which implies an upper bound of pain possible.
For something to experience pain, some information needs to exist (e.g. in the mind of the sufferer, informing them that they are experiencing pain). There are known information limits, e.g. https://en.wikipedia.org/wiki/Bekenstein_bound or https://en.wikipedia.org/wiki/Landauer%27s_principle
These limits are related to entropy, space, energy, etc., so if you further assume the universe is finite (or perhaps equivalently, that the malicious agent can only access a finite portion of the universe due to e.g. speed-of-light limits), then there is an upon bound of information possible, which implies an upper bound of pain possible.