function read() isn't quick enough for reading in ttyS*

Hi all! I'm a novice in Linux

In a program I'm writing in c and, when I use the read() function for reading a byte in the port serial with a configuration: 115200 bps, 8N2

takes from 2ms to 6ms (aleatory, even if a read up to 10 bytes takes the same time), which is too long. (ioctl(fd, FIONREAD, &bytes) last the same)

This is the time is suppose to last:
8 bits: data
2 bits: stop
1 bit: start
time = ((8+2+1)bits/115200bps) = 0.095ms,

then when I read the byte with the function inb_p() takes ~ 0.1ms which is perfect, what I'm needing for my project.

The problem I'm having is that I use signals (with fcntl(fd, F_SETFL, O_ASYNC) ) but this signals last the same time in occur that the read() (unacceptable) so I guess that this is a problem of the driver that is to slow.

So the question is, is there any way to improve the performance of the driver or using read? (doing inb_p is very nasty). Do I'm doing something wrong?

Thanks in advance!, I expect as soon as possible any answer

PD: sorry for my English,