I will try to answer your question but it is quite broad because you have not stated what mode you are talking about S21, Spectrum analysis of Field strength. In some measurement modes the selection of the maximum number of possible display points slows down the measurement much more than speed selection.
I can't accurately answer the question for S21 but the speed is most dependent on the number of display points you select. I did a test on S21 over 7GHz maximum span
With 450 displays - Slow is about 1 Sec /sweep and the others settings are faster?
With 3150 display points - Slow is 7 sec/swp, medium is 5.5 /swp and fast 4 sec/swp.
In Field strength mode.
Judging from the display fluctuations, SLOW is about one update per second, MEDIUM seems to be at least five per second and FAST could be ten or more updates per second by visual indication.
This is better to analyse the affect of the speed settings as you can get a sweep speed readout indication in time per division on the display.
Using spectrum analysis mode and the widest bandwidth of 30kHz, I set the centre frequency to 1GHz and the Span to zero. This is equivalent to the Field strength mode but you can set sweep speed. Changing the bandwidth or the number of measurement points did not affect the speed.
The range of settings selected by toggling the sweep button is from 20mS/div to 1 Sec/div.
Using normal spectrum analysis mode, at 1GHz with a span width of 10MHz, the sweep speed is dependent on the number of display points and the bandwidth setting.
For RBW 1kHz and max display points of 3150 results in a sweep time of around 12.8 Seconds.
For RBW of 30kHz and the lowest resolution setting of 450 the sweep time is about 1 second.
The slow medium and fast settings do not seem to have an effect.