Is Steam Link more demanding on your PC?

I am thinking about buying a steam link. If I stream a game from my PC to TV using it, will my PC have to do much more work? I heard that it still displays the game on your monitor as well as the TV. Will that cause my PC to do a lot more work?

The impact depends on a few things:

• PC
• home network
• TV

In general you shouldn't feel a difference, unless one or more of the components bottlenecks the connection (Slow TV or network, old PC, ...). Also like mentioned in the comment some games may cause problems (e.g. vsync is disabled while streaming), but that's pretty much unpredictable.

The PC itself will not feel much of an extra impact, unless you have a bad graphics card or an old PC altogether. Yes, it will have some extra work, if the game is displayed on both your monitor and TV as the GPU will have some extra load. Anyway, since the game is only rendered once and then duplicated the impact should still be low enough to not or hardly feel it.