I ran some speed tests to compare CASA's speed when imaging single channels vs blocks of channels. The tests were run with savemodel='none' on a lustre node

'speedtest_nchan1_concat': 249,
'speedtest_nchan1_individual': 523,
'speedtest_nchan5_concat': 803,
'speedtest_nchan5_individual': 916,
'speedtest_nchan20_concat': 2303,
'speedtest_nchan20_individual': 2346,
'speedtest_nchan40_concat': 4010,
'speedtest_nchan40_individual': 4136,
'speedtest_nchan80_concat': 8101,
'speedtest_nchan80_individual': 9298,

The fit is y = 98x + 237

So at least until swapping happens, there is no serious advantage or disadvantage to running a different # of channels simultaneously.