While I've not tried it at 4K yet (as my GPU and monitor don't support it well), at 1080p, it doesn't have a noticeable performance hit on the host PC for me, and the video comes out at great quality (high bitrate and no major compression artefacts, much better than FRAPS, for example, and with much smaller file sizes). The only issues I've had are with streaming to laptops, and that's because I'm using a laptop to emulate a shield tablet with limelight. Even that works pretty well, just I've not found a good balance to get a high framerate (as the laptop struggles to decode the stream fast enough, as it's doing it in Java and limelight isn't in an optimisation phase yet - they're just trying to make it work).
OS: Windows 10 64 bit Professional
CPU: AMD Ryzen 5900X
RAM: 48GB
GPU: Radeon 7800 XT
CPU: AMD Ryzen 5900X
RAM: 48GB
GPU: Radeon 7800 XT