As far as I know sed will still read and write the full file, even when you do a modification on a specific line. I've target deleted 1 line with sed and it took quite some time too.
There's a big difference between "quite some time" and 30 minutes, though. Of course sed needs to read the file and write it back to disk, but that's capped by I/O speed which is very high on modern drives - we're talking seconds for a file in the tens of gigabytes on the fastest SSDs, a very far cry from half an entire hour.
This was run on an AWS gp3 SSD EBS volume with 3k IOPS. It was a CPU optimized c6i.xlarge machine. The file in question was 200 GB.
I used "quite some time" because this happened months ago and I don't recall the exact time on the delete. I don't think deleting a line was any faster than doing a find / replace on 1 line. If sed is reading and writing the file in both cases I'd expect both to have the same performance.
