In most cases, acting on <sendbuf> only should be enough, without touching <MSS>. Try to increase sendbuf as much as you want (up to 2147483647) and see if the situation improves.

Here is how it works.

By default, Lightstreamer keeps a tiny sendbuffer (1600 bytes). This allows the server to implement very fast detection of network congestion and adaptively throttle the data flow. What happens with a small sendbuffer is that the congestion windows is kept small as well. This works great with normal streaming activity. But if a huge amount of data needs to be delivered in a short time (similar to a normal download activity), then such a small sendbuffer would slow down the data transfer a lot. That's why increasing the sendbuffer, possibly to very large values (for example, try 100,000,000 bytes), will boost download performance.