Callbacks are one reason UA is slower, winxp home, perl v5.8.7, LWP 5.819 (libwww-perl-5.821), GNU Wget 1.11.4

D:\>perl lwp-bench.pl

LWP::Simple took   2.376 seconds
Size: 64230
Transfer rate: 27036.45 bytes/sec

Win32::I took   1.358 seconds
Size: 64230
Transfer rate: 47312.34 bytes/sec

LWP::UA took   2.717 seconds
Size: 64230
Transfer rate: 23640.49 bytes/sec

D:\>perl lwp-bench.pl

LWP::Simple took   1.327 seconds
Size: 64230
Transfer rate: 48385.75 bytes/sec

Win32::I took   0.396 seconds
Size: 64230
Transfer rate: 162095.51 bytes/sec

LWP::UA took   1.878 seconds
Size: 64230
Transfer rate: 34205.63 bytes/sec

D:\>perl lwp-bench.pl

LWP::Simple took   1.457 seconds
Size: 64230
Transfer rate: 44091.30 bytes/sec

Win32::I took   0.535 seconds
Size: 64230
Transfer rate: 120084.79 bytes/sec

LWP::UA took   2.035 seconds
Size: 64230
Transfer rate: 31564.64 bytes/sec

D:\>



D:\>wget -c http://www.mirrorservice.org/sites/ftp.funet.fi/pub/langua
+ges/perl/CPAN/authors/id/J/JD/JDB/Win32-In
ternet-0.084.tar.gz
--2008-12-03 23:29:27--  http://www.mirrorservice.org/sites/ftp.funet.
+fi/pub/languages/perl/CPAN/authors/id/J/JD/JDB/Win
32-Internet-0.084.tar.gz
Resolving www.mirrorservice.org... 212.219.56.135, 212.219.56.138, 212
+.219.56.139, ...
Connecting to www.mirrorservice.org|212.219.56.135|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 64230 (63K) [application/x-gzip]
Saving to: `Win32-Internet-0.084.tar.gz'

100%[=================================================================
+=============>] 64,230      68.5K/s   in 0.9s

2008-12-03 23:29:29 (68.5 KB/s) - `Win32-Internet-0.084.tar.gz' saved 
+[64230/64230]


D:\>rm Win32-Internet-0.084.tar.gz

D:\>wget -c http://www.mirrorservice.org/sites/ftp.funet.fi/pub/langua
+ges/perl/CPAN/authors/id/J/JD/JDB/Win32-In
ternet-0.084.tar.gz
--2008-12-03 23:29:38--  http://www.mirrorservice.org/sites/ftp.funet.
+fi/pub/languages/perl/CPAN/authors/id/J/JD/JDB/Win
32-Internet-0.084.tar.gz
Resolving www.mirrorservice.org... 212.219.56.135, 212.219.56.138, 212
+.219.56.139, ...
Connecting to www.mirrorservice.org|212.219.56.135|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 64230 (63K) [application/x-gzip]
Saving to: `Win32-Internet-0.084.tar.gz'

100%[=================================================================
+=============>] 64,230      59.2K/s   in 1.1s

2008-12-03 23:29:39 (59.2 KB/s) - `Win32-Internet-0.084.tar.gz' saved 
+[64230/64230]


D:\>rm Win32-Internet-0.084.tar.gz

D:\>
[download]

maybe you can try LWP::Curl next?