Go sets the default buffer size of na new bufio.Scanner to 4096 bytes. I've scanned a ~155MB file with an average line length of 62 bytes with smaller and larger buffer sizes. Here are the results running on a 2019 x86 MacBookPro:
file size: 155605069
lines: 2501619
avg line length: 62.20
1024: 182.103886ms
2048: 116.351501ms
4096: 85.373947ms
8192: 69.776855ms
16384: 62.339557ms
32768: 56.198547ms
65536: 53.285957ms
The results were fairly consistent between multiple runs.