Propagation delay is the time it takes for a signal to propagate through a logic gate or flip flop. In the context of counter design, the propagation delay of flip flops plays a significant role in determining the speed of operation, especially in asynchronous (ripple) counters.

Asynchronous counters are designed using a series of flip flops, where the output of one flip flop serves as the clock input for the next flip flop in the chain. Each flip flop introduces a certain amount of propagation delay, which is the time it takes for the output of the flip flop to stabilize after a change in its inputs.

The main reason why the propagation delay of flip flops affects the speed of operation in asynchronous counters is due to the ripple effect. When the input to the first flip flop changes, it takes some time for the output of that flip flop to stabilize. This delay in stabilization propagates through the subsequent flip flops in the chain, causing a delay in the counting sequence.

The propagation delay can lead to several issues in asynchronous counters:

1. Skew: The propagation delay of flip flops can cause a skew in the counting sequence. Skew refers to the difference in arrival times of the clock signal at different flip flops. This can result in an inconsistent and unpredictable counting sequence.

2. Glitches: The propagation delay can also cause glitches in the counter output. A glitch is a temporary and unwanted change in the output of a logic circuit. Due to the ripple effect, glitches can occur when the outputs of the flip flops are changing during the stabilization period.

3. Limited frequency of operation: Asynchronous counters have a limited operating frequency due to the cumulative effect of propagation delays. If the input clock signal has a high frequency, the propagation delays can accumulate and result in errors or unpredictable behavior in the counter.

On the other hand, synchronous counters, both up and down, do not suffer from these issues because they use a common clock signal to synchronize all flip flops simultaneously. The propagation delay of individual flip flops still exists but does not affect the counting sequence or introduce glitches, as all flip flops are updated at the same time.

In conclusion, the propagation delay of flip flops largely affects the speed of operation in asynchronous (ripple) counters due to the ripple effect, which can lead to skew, glitches, and limitations on the operating frequency.