How slow? On my 2013 MBP, `gsed` (sed from coreutils) can do a replacement like that at about 350 MiB/s (of which most seems to be spent writing to disk, since writing to /dev/null hikes it up to 800 MiB/s).
It was sed substitute command on a ~800Mb file on Thinkpad T470 with SSD. It was taking around 40-50 sec for each substitution. Though as others have pointed, it may not be directly related to article in discussion.
>It was taking around 40-50 sec for each substitution.
Substitution should not be really a relevant metric as it wouldn't influence the result much. Sed/Awk will still have to go through the whole file to find all occurrences they should substitute (and when they do find an occurrence, the substitution would take nanoseconds).
The size of the file is a better metric (e.g. how many seconds for that 800mb in total).
Also, whether you used regex in your awk/sed, and what kind. A badly written regex can slow down search very much.