Found with the superlinear performance fuzzer.
possible non-linear behaviour found
pattern: {"prefix":"","repeating_pattern":"!-- <","suffix":""}
score: 22647.09279001435
[(546.0, 2930621.0), (1092.0, 11428176.0), (1638.0, 25537787.0), (2184.0, 44715325.0), (2730.0, 69763013.0), (3276.0, 100590096.0), (3822.0, 137034665.0), (4369.0, 182348198.0), (4915.0, 239594874.0), (5461.0, 294327263.0)]
i.e. the input is !-- < repeated N times. Starting with < doesn't seem to be quite as bad (?)