Sampling and serial transfer
Probably the serial port is too slow:

Assuming 250 kbits/s, is (including 3 start/stop bits, maybe 2 in your case) in about 250000 (bits/s) / 11 (bits/byte)= about 22 KBytes/s
Assuming each sample has 3 or 4 digits and an 'x' is added (which is 5 characters), 22 (KBytes/s) / 5 (digits/sample) = about 5-6 Ksamples/s you can transfer, which is what you experienced.

So you have to increase the speed for the serial port.