There's currently no way to disable the weak hashing on the hashing side. The control that is there controls the use of it on the syncing side.
Indeed, and I have that same situation. But I think what we may end up waiting for is the IO system calls and so on, which in Go can mean a bit more context switching and stuff than usual. One of those per 128 KiB block, during which time no hashing happens, is probably the majority of the remaining loss.
I had a couple of branches a long time ago that did this concurrently - one routine to read data and put it in a queue, another one or several that hashed data from the queue. This was not a win at the time due to the overhead of managing that and the locking and communication it needs - the current hashing loop is quite well optimized to run fast and never allocate etc for a single file.
But it does mean we're limited to a single core for a single file, while running two hashing algorithms. It might be worth it to try again and parallelize it - but I think it's quite far down on the current list of things that need to happen.