Just shows how good a driver Alonso is to get that time with that terrible engine! Jesus, someone give that man a decent car!
There is nothing impressive about it, not in the sense that a 1:26 isn't good, though it absolutely isn't, the problem is almost everyone behind him had problems.
First Alonso did that time very late, long after everyone else started doing long runs. Vandoorne's time was done not far off Alonso's earlier time and Vandoorne was 1/10th ahead of Alonso or so. He did a half second faster on another run another half hour later on a faster track. He wasn't meaningfully faster than Vandoorne at the same time on track. I think Alonso was the sole guy on track trying to do a fast lap that close to the end, which moved him unnaturally higher up the pecking order. Everyone could probably have gone half a second faster at that point.
Then we have comparing Mclaren to the rest... it doesn't look good. Palmer spun his car in the final corner and missed most of the session and Kmag's car was in the garage most of the session, but both their team mates were half a second up on Alonso's late late fast lap, a second up on Alonso's lap done at similar times, so there is no reason to think both wouldn't be 0.5-1 second ahead of the Mclaren's without issues. Massa's gearbox went and Ocon is way behind his team mate also.
Basically adjusting for issues and when the times were done, ALonso was slower than Vandoorne and the Mclaren's were slower than Ericsson's Sauber. The only two they looked like they might beat are Wehrlein and Stroll who both looked slow today. If Mclaren upped their engine at all at this point, they should be extremely worried. They also didn't do any significantly long runs so god knows how they'll do in a full race.
Last year Ham gained 2 seconds between Fp3 fastest time and pole. If most of the grid does that, well, we've got to see where Mclaren place once everyone is turned up basically.