This won’t take long. Promise.
Here’s the thing:
zgrep -i -e 'dn: uid=Y12345' mother-of-all-ldifs.gz
It really is a time saver, you know. Nice thing about the output is that it’s free of the usual grep-talk (you know, when it carries on about this or that thing being a directory and un-searchable).
You can accomplish the same thing with the “traditional method” (or at least one of them):
zcat mother-of-all-ldifs.gz | grep -i 'dn: uid=Y12345'
… but then you’ll have to deal with all that noise in your output.
In experimenting with this my impression is that zgrep is a little slower than the “traditional” method, but if you’ve got over 100 user entries to find in a half dozen or more backup files you’re not going anywhere soon anyway.