Dirk,
Hey Kirk, yeah if I use larger Text option for the 27" Studio Display the browsing is faster but it looks strange :-).
The UI elements will appear to be 72 DPI in size (yet smoother), and the images will be at full resolution. The display mode on the far left and far right are the only two native resolutions on your display. The others have to be scaled and this scaling slows down the updating of the display.
It seems nearly as fast as only on the MBP
It should be close, but the larger the image the longer it takes to prepare for display and to actually draw to the display. Scaled display modes are much slower.
Also, based on your first screenshot, your MBP's built-in display would get faster with either of the two modes on the far left and right of the display settings.
-Kirk