Syntax for sed .. altered?

[Fergus Daly]

For ages I have been able to run through all the files under a directory changing occurrences of string1 to string2 with the command
find dirname -type f | xargs sed -i 's/string1/string2/g'
It used to take no time at all for say 6000 files.
Now the same command just hangs. 
The files are all text files, no binaries or anything awkward. Just don't understand it.
(By contrast
find dirname -type f | xargs md5sum
still works just fine. 6000 files in less than a second.)
Fergus 

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