Reading from stdin significantly slower than reading file directly?

[methonash]

Hi,

Relative beginner to D-lang here, and I'm very confused by the 
apparent performance disparity I've noticed between programs that 
do the following:

1) cat some-large-file | D-program-reading-stdin-byLine()

2) D-program-directly-reading-file-byLine() using File() struct

The D-lang difference I've noticed from options (1) and (2) is 
somewhere in the range of 80% wall time taken (7.5s vs 4.1s), 
which seems pretty extreme.

For comparison, I attempted the same using Perl with the same 
large file, and I only noticed a 25% difference (10s vs 8s) in 
performance, which I imagine to be partially attributable to the 
overhead incurred by using a pipe and its buffer.

So, is this difference in D-lang performance typical? Is this 
expected behavior?

Was wondering if this may have anything to do with the library 
definition for std.stdio.stdin 
(https://dlang.org/library/std/stdio/stdin.html)? Does global 
file-locking significantly affect read-performance?

For reference: I'm trying to build a single-threaded application; 
my present use-case cannot benefit from parallelism, because its 
ultimate purpose is to serve as a single-threaded downstream 
filter from an upstream application consuming (n-1) system 
threads.