Ladder logic scan time is about 10 microsecond per step, whereas TBASIC is approximately 100 to 500 microsecond per command depending on the command type and number of parameters. There is no simple way to compute the scan time since most TBASIC are executed in {dCusF} which is a differentiated instruction and so is not executed every scan of the ladder program.
If you need to handle fast I/O response, use the interrupt inputs or use them as high speed counter or Pulse Measurement input.