Strange issue on Raspberry Pi UART (with FTDI USB  RS-232 adapter)
The RPi has only one USB Bus. One second long gaps sould be very rare, but you would see them if your data source for cat * uses the same USB bus to tranfer data into the RPi at the same time.

Something with more buffer like 
dd if=/dev/zero of=/dev/ttyUSB0 bs=1M should show less gaps, unless there are more USB devices active.