Computationally less expensive appoaches to simulating reverberation usually try to separate the early echoes and the reverberant tail.

There are a number of different main block structures. One of the common ones looks like this :

Usually the circular buffer blocks will repeated 5 to 10 time instead of the two shown here. Additionally, the allpass filter will usually be chained 5-10 deep as well.

Each of the the circular buffers above has a unique length and time delay.