Hi, I average just under 19 seconds and I have some tips for you on how to become sub-20. I will need a little more information about how you solve and what you are good at and what you are not.
First, and I think most importantly, try to become color neutral. Color neutrality helps your recognition and if you get an easy scramble that's not on white, it will make it much faster.
Second, Learn full PLL. Full OLL is not necessary to become sub-20 and I know this because I know about 40/57 OLL's and I average 18.80.
Third, plan your whole cross out in inspection and if possible, look for your first pair. Also use 9 moves or less to make your cross.
Fourth, practice other events related to 3x3. As an example, 4x4. 4x4 will help your recognition abilities especially during edge pairing and there is also the 3x3 stage at the end if you use Reduction.
I hope this helps, I will be able to give you more in-depth information if I know a little more about what you're good at and what needs improvement. Bye.