For this test i created two almost identical streams. The only difference between the streams is the field length of 10 NVarchar fields. The first stream has fields with a length of 25. The second stream has fields with length 250.
Both streams wite data to /dev/null to limit the influence of IO to a minimum.
Stream 1 writes data with 154,000 rows per second and stream 2 writes with 135,000 rows per second so stream 1 is 13% faster.
The impact may vary on your jobs but if performance is key, it helps to limit the fieldsize if possible.