Quadratic Signature Hashing Problem

What is the quadratic signature-hashing problem in a single sentence:

"The work required to verify a transaction increases by the function O(n²) (i.e. quadratic scaling) based on the number of inputs."

Certain transactions can be designed to include a large number of inputs or outputs that require miners and fully validating nodes to perform a significant amount of CPU-work to verify (Figure 4). For miners, this verification time can delay their work on the next block, placing them at a disadvantage. The worst case scenario would be a transaction where the verification time exceeds the 10 minute target for mining the next block. This can be used to launch a type of denial of service attack against miners and the network at large, as fully validating nodes will not propagate the block without first verifying it completely, unless they mine without validating transactions and undermine the consensus rules of the network. This attack vector is exacerbated with larger blocks, which can accommodate more or larger malicious transactions taking advantage of quadratic signature hashing.