Kimi Raikkonen

Raikkonen seemed to be finding his form of old towards the end of last season, particularly after the news of his new contract had been announced.
However, this season he has started poorly, being outqualified both times by team-mate Sebastian Vettel and comprehensively blown away in these two races. It looks like Raikkonen is declining, and that his pace is on the wane. He may have struggled with some understeer with the car, and he doesn't seem to be particularly at ease with the setup of the car.
He needs to improve his form, however, starting from the next race in Bahrain, otherwise if not, questions will start to be asked about his 2018 seat with the team as his contract expires at the end of the year, if his results don't improve, will Ferrari be keen to renew his contract?
Also, he has been publicly criticised by Maurizio Arrivabene and Sergio Marchionne, two members at the top of the Ferrari hierarchy, so the pressure is clearly on Raikkonen to improve.
Jolyon Palmer

Many argue Palmer was lucky to retain his seat at Renault this season after he struggled last season, although I do believe that he deserved his retention after improved performances towards the end of last season, especially against his team-mate Kevin Magnussen.
However, he has started this season poorly, crashing in Practice 2 in Australia, which affected his performance throughout the weekend, as the car was harder to drive and he retired with brakes problems during the race. In China he was once again outqualified by Hulkenberg, and he failed to get out of Q1, or be it he was caught out by the yellow flags caused by Giovinazzi on the start/finish straight. Also, his gamble to pit for slick tyres at the end of the formation lap failed to pay off, as he spun off, and he finished behind Hulkenberg, despite him having a 15-second time penalty for two separate infringements.
Palmer needs to step up his game, starting from Bahrain, if he is to keep his seat with the team for another year and help Renault move up the grid.
Lance Stroll

Much was made of the decision by Williams to hire Stroll instead of Massa who was at the time scheduled to retire - it turned out that he would be Bottas' replacement, as Massa came out of an extremely short retirement, and Stroll has jumped straight from Formula 3 to Formula 1. Many critics labelled him as a "pay-driver", after the substantial amount of backing that his father brings. Therefoe Stroll was under pressure to perform from the beginning.
He has struggled to prove himself thus far in Formula 1, and crashed in Practice 3 in Australia, which cost him a 5-place grid penalty for a gearbox change. He was well off Massa's pace in qualifying and didn't even get out of Q1. He was also off his pace in the race before he had to retire with a brake issue.
In China, he seemed to improve, and made it to Q3. However, his race ended after just a single lap after a collision with Sergio Perez. Stroll is admittedly still finding his feet in Formula 1, and there were signs of some progress in China, but will have to step up, starting from Bahrain in order to defy his critics and prove that he does belong in Formula 1.