More likely it's the MTU size, especially if your connected via DSL. You can adjust it under the interface (your LAN interface).
(config-if)#ip tcp adjust-mss ?
<500-1460> Maximum segment size in bytes
To find the correct size, ping from one side to the other with a large packet size and decrease it until it is successful. That should be close to the size you enter above. Here's an example from Windows.
F:\>ping -l 1500 -f 192.168.5.15
Pinging 192.168.5.15 with 1500 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Ping statistics for 192.168.5.15:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
F:\>ping -l 1430 -f 192.168.5.15
Pinging 192.168.5.15 with 1430 bytes of data:
Reply from 192.168.5.15: bytes=1430 time=8ms TTL=55
Reply from 192.168.5.15: bytes=1430 time=8ms TTL=55
Reply from 192.168.5.15: bytes=1430 time=10ms TTL=55
Reply from 192.168.5.15: bytes=1430 time=16ms TTL=55
Ping statistics for 192.168.5.15:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 8ms, Maximum = 16ms, Average = 10ms
Hope that helps.
