I’m not sure if you’re being sarcastic but this is not true. Electromagnetic waves travel fastest in a vacuum, so the presence of air would slow it down very slightly and I would expect higher air pressure would slow it down further again only incredibly slightly because the electromagnetic waves would be traveling through a medium less efficient and more different than a vacuum.

Of course I’m making an assumption that you were using wireless signals. For all I know, you could have some weird acoustic link in which case you’d be absolutely right.