To make this clear once and for all:
There is no latency difference between DDR3 and GDDR5 when you just look at the bare memory chips itself. The latency difference between DDR3 and GDDR5 comes from the different memory controllers and the different scenarios of usage in PCs. DDR3 is usally used for CPUs while GDDR5 is used for GPUs.
The main task of GPUs is rendering which requires a lot of bandwidth. GDDR5 is a high performance RAM-type that is able to outperform DDR3 easily in a typical rendering scenario. To make sure that the maximum bandwidth is available most of the time, GPU memory controllers combine many memory accesses to bursts. This is bad for the latency and great for the bandwidth, but since GPUs don't need latency for rendering it doesn't affect performance.
CPUs don't need a lot of bandwidth since they're dealing with computing. DDR3 delivers enough bandwidth for any modern day CPU, so GDDR5 would be overkill for computing. The computing tasks of a CPU are are extremely latency-critical. That's why memory controllers for CPUs work in the complete opposite way as GPU memory controllers. Instead of burst accessing the RAM, you'll make sure that every memory access can happen as immediate as possible. This will kill bandwidth but it will have a positive impact on latency which will eventually increase the computing performance of the CPU.
What does this mean for the PS4?
The PS4 uses a state-of-the-art heterogenous processor architecture from AMD (the so called "HSA") which combines CPU and GPU in one single chip. To ensure that such a heterogeneous processor can deliver maximum bandwidth for rendering and minimum latency for computing, AMD integrates a special DRAM controller. This DRAM controller allows the CPU memory controller to have low latency access while at the same time the GPU memory controller can burst access the RAM. That's why Sony can go for maximum bandwidth with one big GDDR5 RAM pool without having any headaches because of latency.
