FreeBSD 7.1 as a router - poor download speed, but normal upload

[.kkursor]

Hello everybody! Please help me if you can.
I have a home server built on Gigabyte MiniITX motherboard with VIA C3
800MHz CPU and 512M RAM aboard. The server acts as a file server,
torrent downloader, router and mail and HTTP server.
I have a PPTP connection to my ISP through the integrated network card.
The network configuration is:

[0:04] kkursor at dot.kkursor.ru ~ # ifconfig
rl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu
1500
	options=8<VLAN_MTU>
	ether 00:19:cb:54:c6:15
	inet 192.168.0.26 netmask 0xffffff00 broadcast 192.168.0.255
	media: Ethernet autoselect (100baseTX <full-duplex>)
	status: active
rl1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu
1500
	options=8<VLAN_MTU>
	ether 00:0f:ea:e2:cd:e0
	inet 192.168.158.102 netmask 0xffffff00 broadcast 192.168.158.255
	media: Ethernet autoselect (100baseTX <full-duplex>)
	status: active
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
	inet 127.0.0.1 netmask 0xff000000 
tun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> metric 0 mtu 1500
	inet 195.98.183.20 --> 172.30.96.1 netmask 0xffffffff 
	Opened by PID 73795
tun1: flags=8010<POINTOPOINT,MULTICAST> metric 0 mtu 1500

rl1 is integrated network card and rl0 is an additional PCI NIC which is
inserted into the only PCI slot on the motherboard. Connection to ISP is
done over rl1 using port pptp-client.
The problem is that there is very low download speed. My ISP provides
symmetric 4Mbps line, but download speed is poor - about 1 Mbps. Upload
speed is OK. I called ISP, they asked me to connect WinXP computer and
measure speed. It was normal, therefore there is an issue in my server.
If you could point me in the right direction, it would be wonderful.
Thank you very much for your work!

With best regards and looking forward for answer,
Kirill Sarksyan, Russian Federation