Some drivers can do well without much experience. I'm not saying Lance doesn't have talent at all, He's just not the top drawer, and rather far from it imo. Quick compare with his potential team-mate
Kubica went to F1 straight from WSbR, which he won on first attempt. On his first friday session with BMW he topped the timesheets. Less than 3 months later he was competing in a race as a substitute to Villeneuve. He got DNFed for being 2 kg too light after the race, but 2 races later, in Italy, he got his first podium. With similar experience to what Lance has right now - in his 23th race weekend, Kubica got first Pole Position (which was BMW Sauber first and only PP btw), being only 2 times out of top 10 in the process. Hamilton started his F1 career with McLaren (considerably faster than both 2006 BMW and 2017 Williams) and squeezed 6 PP and 6 2nd places out of it in his maiden season. He got 1 season of GP2 before, though. There's similar story with Alonso - he drove for Minardi in his debut season, so had very little chance of spectacular results, but managed to outpace his teammate by 2.6s (!) twice.
Point is, trully talented driver doesn't need entire season to get near to the car's limit.
Lance has been Williams development/test driver since 2015 and tested with older Williams F1 cars on multiple ocassions too, I can't see a reason, why he shouldn't be compared to other drivers or have his mediocre results excused as a lack of experience by now.
All that being said, I still rate Lance higher than Palmer