Looking at a dataset:

$ wc -l sub-20950/beh/sub-20950_task-*.tsv
     457 sub-20950/beh/sub-20950_task-StopSignal_beh.tsv
  315803 sub-20950/beh/sub-20950_task-antisaccade_beh.tsv
     337 sub-20950/beh/sub-20950_task-antisaccade_events.tsv
     154 sub-20950/beh/sub-20950_task-axcpt_beh.tsv
     241 sub-20950/beh/sub-20950_task-flanker_beh.tsv
  316992 total

If **/*antisaccade_beh.tsv is .bidsignored, validation runs in under a second. If not, it takes several minutes at least (I canceled, so I don't know how long).

I think it's reasonable to type check only the first 1000 lines and assume that the rest are fine. We could make this a configurable parameter exposed to the API and CLI, and allow the current behavior to be accessed by setting the value to -1. 0 would load only the header, and a positive number would load that many rows beyond the header.