We have not experienced any kind of issues or reports from other customers regarding your inquiry.
It is very possible this is a compatibility issue with this device. We recommend contacting the device manufacturer for further compatibility information.
There is actually a similar issue posted. "SOL cannot keep up with serial console at 19200+"
I have used PCI cards and an Ethernet Device Server with the same results. Both these devices work in older Intel mothers boards.
That thread you linked to relates to a Linux installation but if you look on Intels list of supported OS's, your board (along with almost all other recent boards on the list) is not listed as having support for Linux. What OS are you using and what company is the manufacturer of the Serial Adapter board? I know you're probably keen to use one of your 3 PCI slots but as a simpler workaround, have you considered a USB to serial adapter?