Longest-time-verifying block
I'm not completely sure if this is what you are looking for, but it may be.

Here you have a type of transaction that takes 3 minutes to verify, and that was reported back in 2013. (CVE-2013-2292). Moreover, in Sergio Demian Lener's blog you can find a related topic, where he explains how he found a vulnerability that used the FindAndDelete function and OP_CODESEPARATOR to create a transaction that toke up more than 5 hours to verify.