I have a TI-84 Plus CE and TI Connect CE on Windows 10. Once I download a game, I use 7zip to extract it to a folder where I keep games. Next, I open TI Connect CE and attempt to open the file. I always get "bad token" messages, even with assembly shells and programs made for TI-84 Plus CE, unless the program was made by me.
The files I download and the files I make are both .8xp files. When I try to open downloaded programs in Source Coder, the website claims the connectivity is slow despite me having good connection. My own files don't have this issue. Why might the files I download not be working, despite working for others?

This is because you are opening the files as basic programs. Use the 'Actions' menu button and select 'Send file from Computer'. Or just drag and drop the file onto the calculator in TI Connect CE.

More specifically, drag-and-drop the file into the Calculator Explorer pane of TI Connect CE, not the Program Editor pane.