iptables - Traffic redirection with Rasperry Pi router

I use a Rasperry Pi (running raspbian) as a wi-fi router. It redirects traffic from the Ethernet wire (eth0 - wire + ppp0 - L2TP connection, lx2tp used for it) to wlan0 (USB wi-fi dongle, hostapd used for the AP functionality).

And the problem is, some sites are loaded really slow or not loaded at all. Take for instance soundcloud .com - the command curl -L soundcloud.com performs in a snap on the router itself but hangs forever on a laptop connected to it via Wi-Fi. It must be traffic redirection issue.

Here is my /etc/network/interfaces:

pi@pi ~ $ cat /etc/network/interfaces
auto lo

iface lo inet loopback
iface eth0 inet dhcp

iface wlan0 inet static
    address 192.168.1.1
    netmask 255.255.255.0

up iptables-restore < /etc/iptables.ipv4.nat

And here are the redirection rules:

pi@pi ~ $ cat 1.sh
sudo iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
sudo iptables -A FORWARD -i ppp0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT
sudo iptables -A FORWARD -i wlan0 -o ppp0 -j ACCEPT
sudo sh -c "iptables-save > /etc/iptables.ipv4.nat"

Is there something wrong with this setup?

UPD:

Route table at the Pi:

pi@pi ~ $ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         *               0.0.0.0         U     0      0        0 ppp0
10.89.64.0      *               255.255.248.0   U     0      0        0 eth0
85.21.0.0       10.89.64.1      255.255.255.0   UG    0      0        0 eth0
hdns2.corbina.n 10.89.64.1      255.255.255.255 UGH   0      0        0 eth0
192.168.1.0     *               255.255.255.0   U     0      0        0 wlan0
hdns1.corbina.n 10.89.64.1      255.255.255.255 UGH   0      0        0 eth0

UPD2:

Turns out is't some SSL issue (that's from vagrant Ubuntu box on the OSX host):

vagrant@precise64:~$ curl -v -L soundcloud.com/pure_virtual
* Hostname was NOT found in DNS cache
*   Trying 93.184.220.127...
* Connected to soundcloud.com (93.184.220.127) port 80 (#0)
> GET /pure_virtual HTTP/1.1
> User-Agent: curl/7.38.0
> Host: soundcloud.com
> Accept: */*
> 
< HTTP/1.1 302 Found
< Date: Sat, 04 Apr 2015 17:38:06 GMT
< Location: https://soundcloud.com/pure_virtual
* Server am/2 is not blacklisted
< Server: am/2
< X-Frame-Options: SAMEORIGIN
< Content-Length: 0
< 
* Connection #0 to host soundcloud.com left intact
* Issue another request to this URL: 'https://soundcloud.com/pure_virtual'
* Found bundle for host soundcloud.com: 0x7f73d4f2d0b0
* Hostname was NOT found in DNS cache
*   Trying 93.184.220.127...
* Connected to soundcloud.com (93.184.220.127) port 443 (#1)
* successfully set certificate verify locations:
*   CAfile: none
  CApath: /etc/ssl/certs
* SSLv3, TLS handshake, Client hello (1):

* Operation timed out after 0 milliseconds with 0 out of 0 bytes received
* Closing connection 1
curl: (28) Operation timed out after 0 milliseconds with 0 out of 0 bytes received

Answer

It appeared to be an MTU/packet fragmenting issue, solved with this iptables spell:

iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu