The query will scan every row in the table unless the column happens to be the leading column in an index. Scanning large tables can impact performance. You may want to restore a backup of the database to a non-production location and do your research there. It would be advisable to test the fixes there as well, then run CHECKDB to ensure you have resolved the data issue, and then run those fixes in production.
There are no special teachers of virtue, because virtue is taught by the whole community. --Plato