diff --git a/auto_rx/autorx/decode.py b/auto_rx/autorx/decode.py
index 882f320..c3625b3 100644
--- a/auto_rx/autorx/decode.py
+++ b/auto_rx/autorx/decode.py
@@ -531,7 +531,12 @@ class SondeDecoder(object):
             # This fsk_demod command *almost* works (using the updated fsk_demod)
             # rtl_fm -p 0 -d 0 -M raw -F9 -s 307712 -f 1676000000 2>/dev/null |~/Dev/codec2-upstream/build/src/fsk_demod --cs16 -p 32 --mask=100000 --stats=5  2 307712 9616 - - 2> stats.txt | python ./test/bit_to_samples.py 48080 9616 | sox -t raw -r 48080 -e unsigned-integer -b 8 -c 1 - -r 48080 -b 8 -t wav - 2>/dev/null| ./mk2a_lms1680 --json
 
-            decode_cmd = "%s %s-p %d -d %s %s-M raw -F9 -s 250k -f %d 2>/dev/null |" % (
+            # Notes:
+            # - Have dropped the low-leakage FIR filter (-F9) to save a bit of CPU
+            # Have scaled back sample rate to 220 kHz to again save CPU.
+            # mk2mod runs at ~90% CPU on a RPi 3, with rtl_fm using ~50% of another core.
+
+            decode_cmd = "%s %s-p %d -d %s %s-M raw -s 220k -f %d 2>/dev/null |" % (
                 self.sdr_fm,
                 bias_option,
                 int(self.ppm),
@@ -545,7 +550,7 @@ class SondeDecoder(object):
                 decode_cmd += " tee decode_IQ_%s.bin |" % str(self.device_idx)
 
             # LMS6-1680 decoder
-            decode_cmd += f"./mk2mod --iq 0.0 --lpIQ --lpbw 160 --lpFM --dc --crc --json {self.raw_file_option} - 250000 16 2>/dev/null"
+            decode_cmd += f"./mk2mod --iq 0.0 --lpIQ --lpbw 160 --lpFM --dc --crc --json {self.raw_file_option} - 220000 16 2>/dev/null"
             # Settings for old decoder, which cares about FM inversion.
             # if self.inverted:
             #     self.log_debug("Using inverted MK2A decoder.")
diff --git a/auto_rx/build.sh b/auto_rx/build.sh
index f126eee..5779802 100755
--- a/auto_rx/build.sh
+++ b/auto_rx/build.sh
@@ -47,7 +47,7 @@ gcc mp3h1mod.c demod_mod.o -lm -O3 -o mp3h1mod -w $VERS_FLAG
 # Build LMS6-1680 Decoder
 echo "Building LMS6-1680 demod."
 cd ../../mk2a/
-gcc mk2a1680mod.c -O3 -lm -o mk2mod $VERS_FLAG
+gcc mk2a1680mod.c -Ofast -lm -o mk2mod $VERS_FLAG
 
 echo "Building iMet-4 demod."
 cd ../imet/