FWIW: no, this is not true. There is no speed difference between reads and writes for DRAM. You could do a continuous burst of 1 GB of data from or to a DRAM in the same time.But dram memory has much bigger problem with writes than reads.
The thing that can hurt DRAM throughput (among other things) is quickly moving from reads to writes and back, such as read-modify-writes. OTOH, you can avoid most of this penalty by grouping a lot of reads and writes together, which is something a GPU should be good at.