Hard drives can read read data sequentially very, very quickly. On whatever generic SATA hard drive I have in my workstation, I get:

  eklitzke@gnut:~ $ time sudo head -c 1073741824 /dev/sda > /dev/null
  
  real	0m8.267s
  user	0m0.220s
  sys	0m0.810s

So I can read 1G off the drive in about 8 seconds.

Even if the login command does need to sequentially read through logs to find the last login time (and I'm skeptical of that, because that would be a stupid way to implement login), I don't see how that would explain multiple seconds of waiting.