Well, ISA +11° isn’t too uncommon IRL. Some people are seeing a bug where it is ISA +50° or more at altitude. If you let it get much below the optimal speed in the climb, it will take a while to get back. I’d say double check airspeed in the climb and never let it get below 240 kts until transitioning to mach .64 (around 30,000’). When you are getting closer to FL400, vertical speed will probably be down closer to 800 ft / min depending on weight (at ISA+11°). It isn’t exactly a speed demon, climbing at .64 mach.
Other thing I would say is make sure wing / engine anti-ice is off, but I don’t believe the loss in performance is modeled in the sim.