If you keep playing the rhythms found in my harmonica lessons at least 3 times a week, your speed will eventually improve.
Some tips on improving speed:
Do the "tucka-toodle" and "tucka-ticka" rhythms WITHOUT a harmonica, and try to increase your speed gradually.
Breath through your nose while playing the rhythms.
Exhale through your nose while playing a blow, and inhale on a draw.
Keep your mouth and lips completely relaxed while playing rhythms. Breathe from your belly, not your chest. This is really important.