IIRC a fast blink is due to to little current draw from the light bulbs ( burnt out light, LEDs, missing bulb)
Slow blink seems like it would be from the opposite, too much load (added bulbs, bulbs that draw more current than the factory spected, bad flasher unit?, Resistance in the wiring or terminals, wrong flasher unit)
Should be the other way around. The more current the bulbs draw, the more current going through the flasher. The bimetal then heats up faster.
So if the lamps are drawing more current, the blinkers work quicker.
Another reason for drawing more or less current is the voltage across the lamps. Higher the voltage, the more current drawn, therefore quicker blinking.
If system voltage is low at curb idle, you may or may not notice the headlights dim a little but would notice signal flash slowing. Does alternator gauge needle twitch with signal flash?
