After upgrading the kernel (3.2.57-3+deb7u2 => 3.2.60-1+deb7u1) on a server used for NAT, all Windows7 hosts have a very slow download speed using HTTP but only from some servers. The NAT machine has 2 ISP connections (main and backup) and this problem occurs only when using the main one. When switching to the backup link this problem is not encountered.
Fixed by downgrading to 3.2.57-3+deb7u2.