This is mine i9 2.3ghz , 5500m , 16gb, 1tb.
Using 2012 imac as monitor via thunderbolt 2-3 adapter. Both imac and macbook screen resolutions/scaling set to 'Default'. Fans always around 2600rpm on idle. Very annoying. Also tried 4k LG monitor with usb-c hdmi adapter, also usb-c mini dp adapter, same result. On small load, fans go crazy and after the load, it takes 10 mins to fans to go back to 2600rpm. They never return to 1800rpm, which is normal idle fan speed.I suspect it's not about cpu temperatures, but it's about thunderbolt or Platform controller hub die temperatures. Also looks like Windowserver process is always using some cpu.
When not connected to external monitor, everything seems normal. Fans around 1800rpm on idle.