The photo is of a KC-97L, which has 2 underwing pods, each containing a J47 jet engine (used for take-off and for short periods while refueling) - which enabled higher max take-off weight and eliminated (barely) the need for the diving refueling profile.
I have seen cruising speeds for the KC-97 of between 230 mph and 300 mph (the regular C-97 lists 300 mph as well), and the listed max speed is 375-400 mph for KC-97s and C-97s.
I'm sure that the extra underwing fuel tanks and the boom reduced the "best-range cruise speed", but in addition I suspect that deploying the boom required a drop in speed for best control of the boom.
Refueling speeds I have seen vary from 200 mph to "over 240 mph" without mentioning which models of KC-97 are being referenced.