multi curl working very slow when threads > 10

CentOS 5.5 (Linux 2.6.35 x86_64)
Curl: 7.21.3
Php: 5.3.5

I'm trying simple multi curl script (GET list of URLs in 10-100 threads). It
does work on any windows, but when I try the same code on CentOS: if threads
= 10, everything works almost fine, but when I set threads = 50 or 100 the
process is almost stuck, it takes  minutes for "multi_curl_exec", while on
windows this function executed for less than 1 second. System resources are
not loaded, it's dedicated server with 100mbit port. While on windows same
script on busy machine with 6mbit port works as it should - hundred times
faster, loading my network port. I do not post php source, because script is
fine (on windows7 or xp), it works in asynchronous way.

I tried 2 different centos servers in 2 different datacenters - same story.
Is this centos problem? php-Curl on centos problem (as I said same
php-Curl@windows = fine) or what ?

Everything works fine even if I run it through openvpn server on that
centos, so it is NOT a firewall or something.

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