Multivibrator maximum frequency
Try adding a schottky anti-saturation diode between the base and collector of your transistors. With Rb1=Rb2=10k, C1=C2=100pF, Rc1=Rc2=270R and T1=T2=BC547B and BAT42 anti-saturation clamps, I get just under 1MHz on a breadboard. Without the diodes, I can't get much more than about 100-200kHz (using different component values)