Hi Marton,
I respectfully disagree with this approach. Reading the old contents is not just used for backup. It's used so that flashrom can selectively erase and write blocks which usually saves time because erase and write operations are orders of magnitude slower than reads on NOR flash.
So, it's not a bad thing to support this option, but I think there is little benefit and can actually make the process slower in most cases. (Your case might be different, of course)
Nico Huber's unbounded read patch should help reduce the initial read time by a large factor (~90% reduction). Try marking your chip as tested for unbounded reads. You might find that the read and selective erase/write is faster than unconditional erase/write.