I heard that there was an old block with one transaction and it takes 20 seconds of CPU time to verify.
1) Which block was it? I forgot its block height.
2) Is there a block that takes more time to verify? Or is there a similar block?
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bb41a757f405890fb0f5856228e23b715702d714d59bf2b1feb70d8b2b4e3e08 is usually the transaction referred to as being the megatransaction that took seconds to verify. It is the only non-coinbase transaction in block 000000000000000003dd2fdbb484d6d9c349d644d8bbb3cbfa5e67f639a465fe.
Nowadays this transaction is verified much faster than 20 seconds. Last time I benchmarked this, it was ~1 second. With the introduction of
assumevalid in Bitcoin Core 0.14, the transaction's signatures are no longer fully validated so it takes the same time as most other blocks, ~60 milliseconds.
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.