You answered your own question, the easy answer is yes it can.
did it? Answering that would require having an engineer look at the unit, though if it just seems a little slow I doubt it.
Both the modem and the router are designed for the reception of electromagnetic signals (in your modem's case the coax or phone line will act as an antennae), if your pumping enough RF (my guess is that it would be something around the 2.4 Ghz range as that is popularly known to be transferable to water [Yes you effectively microwaved your house {also the frequency your router is designed to receive I might add}]) into your walls to kill insects then I do not find it hard to believe that you are pumping enough RF into your walls to destroy electronics with ease.
That said if something goes pop, you will usually know it, especially with this kind of abuse the failure would normally be catastrophic. If the slight slowdown concerns you you might just want to replace the modem and router for peace of mind, but I wouldn't.
Do me a favor though, tell me how that termite treatment works out, I have never heard of it before (and did not see it on a quick Google) and my guess is that either it will be ineffectual or the FCC and EPA will be going nuts if it becomes popular.