TCP Client very slow

I have written an Chatclient.
At the TCP1 DataAviable() Event I wrote this code:

if tcp1.IsConnected then

//read new data from internal buffer
StringFromServer = tcp1.ReadAll(Encodings.UTF8)

end if

StringFromServer is a public string.

I take a timer1 to manage this StringfromServer all 300 ms.

To try if it works I create a second time (timer2) 2000 ms.
Timer2 send a small “Hello Frome here” to the server.

The sendcode is:

if tcp1.IsConnected then
end if

Then I try this with 1 Browser, all ok.
Second Browser (client) all ok.
But after the Browser all ist horrible slow.
The Browsers are on different Computers.

The Problem ist NOT the server.

What do I wrong?