If your file contains paragraphs (ie. blank lines between reasonable sized block of text, something like this might work. It is pretty quick, processing a 587 MB test file below (replicated several dozen times to get a file of comparable size) in under a minute.

You can adjust the definition of a 'word' to suit. I've specified 1 to 5 words either side to account for the word being at the start or end of a paragraph.

#! perl -sw
use strict;

our $WORDS ||= 1;
our $KEYWORD || die "-KEYWORD=word needed";

local $/ = ''; # Paragraph mode

my $re_word = qr[\S+\s+];

my $re_5w_key_5w = qr[
    (
        $re_word {1,$WORDS}
        \Q($KEYWORD\E[,.;:!?]*\s+
        $re_word {1,$WORDS}
    )
]ix;

open IN, '<', $ARGV[ 0 ] or die $!;

while( <IN> ){
    if( $_ =~ $KEYWORD ) {
        while( $_ =~ m[$re_5w_key_5w]g ) {
            print "'$1'\n---\n";
        }
    }
}
[download]

Results:

[ 0:58:39.10] P:\test>404751 -WORDS=1 -KEYWORD=poverty "Rhetoric - Ari
+stotle.txt"
'or poverty; it '
---
'or poverty but '
---
'or poverty, of
'
---
'or poverty or '
---
'his poverty, a '
---
'in poverty or '
---
[ 0:58:50.09] P:\test>404751 -WORDS=3 -KEYWORD=poverty "Rhetoric - Ari
+stotle.txt"
'to wealth or poverty; it is
of '
---
'to wealth or poverty but to appetite.
'
---
'of wealth or poverty, of
being lucky '
---
'by sickness or poverty or love or
'
---
'disregard of his poverty, a man aging '
---
'by us in poverty or in
banishment, '
---

[ 0:58:53.10] P:\test>404751 -WORDS=5 -KEYWORD=poverty "Rhetoric - Ari
+stotle.txt"
'action due to wealth or poverty; it is
of course true '
---
'due not to wealth or poverty but to appetite.
Similarly, with '
---
'the sense of wealth or poverty, of
being lucky or unlucky. '
---
'are afflicted by sickness or poverty or love or
thirst or '
---
'man by disregard of his poverty, a man aging war by '
---
'who stand by us in poverty or in
banishment, even if '
---
[download]