diff --git a/README.md b/README.md
index 67b0067..68c151a 100644
--- a/README.md
+++ b/README.md
@@ -24,7 +24,7 @@ Special commands built-in, not passed to the ACIA and 6502:
 | ^P | Pause/resume 6502 processor execution |
 | ^R | Reset the 6502 processor (also reset on USB reset) |
 | ^E | Local echo toggle, off by default |
-| ^T | Show clock ticks and instruction count |
+| ^T | Show clock ticks, instruction count, and frequency since last ^T |
 | ^G | Show help |
 
 Intended to be somewhat compatible with [Grant Searle's Simple6502](http://searle.hostei.com/grant/6502/Simple6502.html),
diff --git a/src/Makefile b/src/Makefile
index 8a62758..0b6759e 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -3,7 +3,7 @@ MAKEFLAGS += --no-print-dir
 Q := @
 endif
 
-OPT_FLAGS ?= -O2
+OPT_FLAGS = -O0 -g
 
 CFLAGS += -Wall -Wextra -Werror -Wno-char-subscripts\
 	$(OPT_FLAGS) -std=gnu99 -g3 -MD \
@@ -22,7 +22,6 @@ CROSS_COMPILE ?= arm-none-eabi-
 CC = $(CROSS_COMPILE)gcc
 OBJCOPY = $(CROSS_COMPILE)objcopy
 
-OPT_FLAGS = -Os
 CFLAGS += -mcpu=cortex-m3 -mthumb \
 	-DSTM32F1 -DDISCOVERY_STLINK -I../libopencm3/include \
 	-I .
diff --git a/src/main.c b/src/main.c
index 1c17f4f..7916d19 100644
--- a/src/main.c
+++ b/src/main.c
@@ -84,9 +84,11 @@ static const char *usb_strings[] = {
 };
 
 static bool paused = true;
+uint32_t millis = 0;
 void sys_tick_handler(void)
 {
 	if (paused) return;
+	++millis;
 	step6502();
 	gpio_toggle(GPIOC, GPIO13);
 }
@@ -110,6 +112,7 @@ char *process_serial_command(char b) {
 	} else if (b == '\x10') { // ^P
 		paused = !paused;
 		return paused ? "paused" : "resumed";
+	} else if (b == '\x06') { // ^F
 	} else if (b == '\x12') { // ^R
 		reset6502();
 		paused = false;
@@ -118,11 +121,24 @@ char *process_serial_command(char b) {
 		local_echo = !local_echo;
 		return local_echo ? "local echo enabled" : "local echo disabled";
 	} else if (b == '\x14') { // ^T
+		static uint32_t last_ticks = 0;
+		static uint32_t last_millis = 0;
+		uint32_t hz = 0;
+		if (last_ticks != 0) {
+			uint32_t elapsed_ticks = clockticks6502 - last_ticks;
+			uint32_t elapsed_millis = millis - last_millis;
+
+			if (elapsed_millis != 0) hz = elapsed_ticks * 1000 / elapsed_millis;
+		}
+
+		last_ticks = clockticks6502;
+		last_millis = millis;
+
 		static char buf[64];
-		snprintf(buf, sizeof(buf), "%ld ticks\r\n%ld instructions", clockticks6502, instructions);
+		snprintf(buf, sizeof(buf), "%ld ticks\r\n%ld instructions\r\n%ld Hz", clockticks6502, instructions, hz);
 		return buf;
 	} else if (b == '\x07') { // ^G
-		return "^V=version ^R=reset ^E=echo ^P=pause ^T=ticks ^G=help";
+		return "^V=version ^R=reset ^E=echo ^P=pause ^T=timing ^G=help";
 	}
 
 	return NULL;