Not sure if I've excised this quote properly, but here goes.
I was wondering this also. Shallan addresses it a couple of times in WoR.Calm, Shallan told herself. Be calm!
Ten heartbeats.
But for her, it didn’t have to be ten, did it?
No. It must be. Time, I need time! (WoR ch. 34)
and
She stepped right up to him, ready to summon Pattern. He wasn't like other Shardblades; she acknowledged that now. He could come more quickly than the ten requisite heartbeats. He'd done that before. She hadn't been willing to admit that he was capable of it. Admitting that would have been too much. How many more of my lies, she thought, hold me back from things I could accomplish? But she needed those lies. Needed them. (WoR ch. 88)
So I think we can conclude from this that she has always been able to summon her blade instantaneously, like Kaladin (although Syl said she needed to be there and ready [WoR ch. 86]) and that her constant references to ten heartbeats were down to her confusion and or madness.
It does seem a little forced on Brandon's part. The blade was called "the fruit of her sin, the proceeds of her most horrific act" in TWoK. Having read WoR now, it seems that Shallan is referring to forming her shardblade for the first time to kill her mother (unless there is some other horrific event in her past we are yet to hear about). I'm not sure even an insane Shallan would refer to that as "fruit" or "proceeds". That said, the spren/blade reveal with Syl was pretty damnation awesome and writing the Shallan blade stuff any other way probably might have detracted from that, so I can let it slide.