length of time com port communication

Hello,

I am interested in the time it takes to send a certain signal via a COM port. Of course I can calculate how long it takes to transmit the signal. But how long does it take until the transmission starts? So I thought could simply measure the time by using TIC TOC. But I'm not sure at what point of time Matlab continues processing. Does it wait until the signal has been transmitted? Does it only wait until the signal was written in the buffer?

So has anybody an idea when Matlab continues processing? Hope you understand what I mean.

Thanks!