10.4.4.3.2 Area and Speed in the Soft FIFO Controller

The size and operating frequency of the Soft FIFO design is dependent upon the configuration and optional features that are enabled.

Note:

A single clock design will be smaller and faster; this is because the synchronizers and gray encoder/decoders are not required.
Gray encoders/decoders are generally required (or strongly recommended) in dual-clock designs, especially for things like asynchronous FIFOs. They are not typically needed in single-clock designs.
Port depths that are not a power of 2 will generate a larger and slower design. The reason is that logic optimization occurs for power-of-2 depths. Thus, if you need a 66 x 8 FIFO, it may be more advantageous to select a FIFO depth of 64 or 128 if area and/or speed are concerns.