And it is true, we are unhappy with that becase we need higher performance.
Here are few ideas how to solve it:
1) First, create a rectangular CentiLeo Area Light covering your windows between interior and exterior and make "is portal" flag enabled.
2) Second, try use Metropolis sampler as opposed to Path Tracer.
Metropolis is enabled with "Firefly killer" parameter.
If it is > 0 then Metroplis will be working on indirect lighting and Noise Aware path tracer will be working on direct lighting.
If Firefly killer is close to 1 then it will kill better all the hard and bright hot spots but it can also send to the grave some bright edges of indirect image portion.
If Firefly killer = 1 then it will have no fireflies with more iterations but it will be not a very honest picture. Some value like Firefly killer = 0.5 or 0.8 are very good.
If Firefly killer = 0 then both direct and indirect lighting are computed with Noise Aware Path tracing and spend ~1.5x more samples per render iteration.
See the pictures with Evgeny interior MLT (firefly killer = 0.5) vs Noise Aware Path Tracing (firefly killer = 0):
Here NAPT version computed 8:30 mins and MLT computed the same scene 6:16min (it is cleaner a little bit but looses some few detailes).
3) One more recomendation is for those people with good GPUs. Simply increase render iteration power up to 1. It denotes that every iteration will compute more samples. It will icnrease the indicator of Million samples / second (see in Interactive render window).
4) Wait until a new version with better automatic noise detection for NAPT.