It's built into GNU Radio. Look in <install_dir>/share/gnuradio/examples/dtv/uhd_atsc_rx.grc

However, it requires quite a bit of cpu and it's performance is terrible unless you're close to the transmitter(s) and have high SNR signals.

How does that matter in digital transmissions? In analog, with low SNR results, you'd get fuzzy images and other similar issues. In digital broadcasts, you either receive the data stream or not. If you do, you get a signal. If not, you get nothing. I'm not an expert in TV broadcast engineering, but I'm not aware of TCP/IP "did you get that? no? here it is again" type of system as that doesn't work for realtime/live very well.

I think it has to do with the CPU usage of the error correction. You're right that it either works or it does not, but there is a small bit between working and not working where there is a "can be made to work if the errors can be corrected" region of the signal to noise ratio.

If there is enough CPU after decoding valid frames to error correct invalid frames, and the error correction data is intact enough, AND there is enough CPU to fix the error and decode the frame before the next frame comes in, I imagine that it would succeed. If any of those are not true, it will fail, and require more processing on the subsequent frames until a new I-frame is received.

This is half estimated guess; I don't know the nuts and bolts of how ATSC works, really.

Performance compared to a hardware demodulator in a television or set top box. A low SNR signal that the hardware demodulator can easily handle may not work at all with the GNU Radio software demodulator.

A radio baseband ASIC is much faster than a software one.

yeah, they have lots of transistors dedicated to error correction, and a core i7 does not.