RN A

I've been a nurse for 2 years on a medical/surgical unit and have recently transferred to the ICU/CCU. I am currently working with a nurse preceptor for my orientation to the units. My preceptor is great and very knowledgable, but she seems to stress out over everything, both critical and minute. While I draw blood from PIVs/Central lines, she breathes down my neck saying that I need to hurry and that I'm going to slow/will cause a blood clot for the patient. Now, I understand the importance of drawing labs quickly and efficiently followed with a good flush (I've done this skill in my previous position), but my preceptor is the first nurse I've met who freaks out about leaving blood in the line for the few seconds that it takes to fill up your lab tubes with the transfer device. I've done some research online and find absolutely no info about clotting time within an IV line. Can leaving the blood in the IV line for the amount of time it takes to transfer the blood from the syringe to the lab tubes really develop a clot and cause damage to the patient? I feel like I'm going as quickly as I can and she expects me to be flushing my line while simultaneously filling my lab tubes...Is it necessary to be this rushed?