Now that F1 testing is over here is what I've learned by spending 6 hours per day watching the live stream.
Ferrari has stolen a march on everyone and is roughly .3 quicker than Mercedes, who sandbagged through most of the test until the last day. Ferrari has had a few small issues such as electrical and hitting something on the track which both caused Vettel to stop, in one case against the barrier. The car is easy to drive and LeClerc is also very quick.
Mercedes is fast and after the first week introduced a change by changing out over 1500 new parts, mostly aero parts over last weekend. It has made them better.
the field is bunched quite close behind. Red Bull and Renault are both about the same but only .1 or .2 slower than Mercedes. Then there is a group of three teams right behind them, another .1 behind. Torro Rosso, Alpha, MacLaren are all very good and showed a lot of quickness.
Following them is Haas, then racing point about another .1 back and finally Williams another .5 seconds after that.
Williams did not get the car running until day 3 1/2 and so they were way behind. the car improved significantly but they are still over a second back on Mercedes.
Now you can't read too much into testing. the teams all have an MO and most of it means they do not focus on lap times. The last day they work on qualifying trim and so there is some correlation. Williams tried to catch up and did to some degree. Time will tell. They are putting on a brave face,But they haven't shown a lot of speed. Everyone else is going faster by at least a second.
So bottom line it will be very close in the mid field with all the teams making big strides and the cars are a bit faster than last years cars. Alpha, toro Rosso and Renault will be right there if the big boys falter.