A quick 3000T datapoint (include disclaimer on methodology here):
The 3000T 1Mwfms/s starts dropping around 20ns/div. While at 1Mwfms/s, if enabling Averaging, the wfms/s drops to 26kHz (at least according to the trigger output), regardless of the # of averages (which is 2-64k on the 3000T).
Below that, there is a very variable ratio between Normal wfms/s and Averaging wfms/s (in the 1-50 range). The ratio gets smaller as longer time bases, with an asymptote at 1.
At and above 20us/div, the sample rate is locked at 5GSa/s in both mode. Below 20us/div, the Normal sample rate starts dropping and the Averaging sample rate is locked at 1/4 the Normal sample rate (as reported by the scope).
In all cases, the number of averages (2-64k) has no impact on wfms/s.
Rough wfms/s (as measured by Trig Out - disclaimers apply):
500ps/, 1ns/, 2ns/, 5ns/, 10ns/: 26.4 kHz
20ns/: 34.2 kHz
50ns/: 13.8 kHz
100ns/: 6.9 kHz
200ns/: 1.7 kHz
500ns/: 1.1 kHz
1us/: 1.1 kHz
2us/: 1.0 kHz
5us/: 950 Hz
10us/: 820 Hz
20us/: 650 Hz
50us/: 96 Hz
100us/: 96Hz
200us/: 62 Hz
500us/: 78 Hz
1ms/: 62 Hz
2ms/: 44 Hz
5ms/: 18 Hz
10ms/: 9 Hz
20ms/: 4 Hz
50ms/: 2 Hz
100ms/: 1Hz