I solve my F2L on the left, and I can get 15-17 second averages. I've been thinking if my F2L times will lower to less than 10 seconds if I switch to cross on bottom, but that would take a long time to get used to. I average around 20 seconds with cross on bottom.
I average 11 seconds on F2L with cross on left (including cross). What should I do?
You have three options.
1. Keep cubing with cross on left. Tomasz does this.
2. Switch to cross on bottom. It will take some time to get used to.
3. Use a different method, preferably one that doesn't use a cross.
Out of all options, number 1 seems the best to me, for you.