Check the LEGS page and you will see that the FMC actually wants a speed lower that 210, it will probably be 170 or 150 knots.
The descent page will have 210/FLAPS because it cannot bring the speed back below the Flaps UP speed. The FMC descent plan has a slow down for the next flap setting. If you hold onto 210 and delay the Flap you keep excess energy above the FMC calculated PATH.
In Approach mode the FMC has calculated a descent path built from the runway threshold backwards. It is a glide path. The aircraft will drive to follow the GP, With any excess energy carried into the approach plus putting out the flap "late" the aircraft is now fast and trying to follow the GP so will be a struggle to make it work.
The options are select Flaps as soon as the Command Airspeed Bug gets to 210 and accept a slower approach. Or just disconnect, ignore the PATH and look out the window and fly her like a Legacy B737.
Either way works.