Improve Powershell Performance to Generate a Random File

I'd like to user Powershell to create a random text file for use in basic system testing (upload, download, checksum, etc). I've used the following articles and come up with my own code snippet to create a random text file but the performance is terrible.

• Generating Random Files in Windows (stackoverflow.com)


• PowerShell – Creating Dummy files (verboon.info)


• Create large files with Powershell (chris-nullpayload.rhcloud.com based on verboon code above)

Here is my code sample that takes approximately 227 seconds to generate a 1MB random text file on a modern Windows 7 Dell laptop. Run time was determined using the Measure-Command cmdlet. I repeated the test several times during different system load with similar long runtime results.

# select characters from 0-9, A-Z, and a-z
$chars = [char[]] ([char]'0'..[char]'9' + [char]'A'..[char]'Z' + [char]'a'..[char]'z')
# write file using 128 byte lines each with 126 random characters
1..(1mb/128) | %{-join (1..126 | %{get-random -InputObject $chars }) } `
  | out-file test.txt -Encoding ASCII

I am looking for answers that discuss why this code has poor performance and suggestions for simple changes I can make to improve the runtime for generating a similar random text file (ASCII text lines of 126 random alphanumeric characters - 128 bytes with "\r\n" EOL, output file an even number of megabytes such as the above 1MB sample). I would like file output to be written in pieces (one or more lines at a time) so that we never need a string the size of the output file stored in memory.