this is in senarios.
same AC
same ord
same fuel.
A film would help, otherwise were throwing ideas at the wall. The screen shot with the K4 doesnt show us how much time you've allowed at full throttle for it to accelerate up to its current speed or your processes in getting there... ontop of that, things look fine....
Lets disect your screenshot: First, you're actualy doing (true AS/red needle) ~385 mph in your screenshot, with only an indicated (big white needle) of ~327. You can also verify/cross-check with the E6B and the speed/altitude it gives you. (we can elaborate more on true and indicated airspeeds, ala the two needles your seeing/misinterpreting on the instrument, if you would like/need.... ). Second, references -

As you can see for yourself, all is operating fine... your K4 isn't going slow at all....
(Next opportunity, try to film or screenshot during a problematic buff run)
In scenarios usualy the formations dont fly at full throttle, so you should be able to catch up using full throttle (which means then that you're definetley doing something differen't than they are to generate so much drag you cant catch up at full power). If, however, they are at full power then nothing you will do is going to make you go fast enough to catch up unless you took less fuel or ditch some ord and make yourself lighter.
Flaps are down on some bombers at spawn (the K4 spawns with them up), unless you retract them they will remain down possibley throughout the flight. Open bomb bays generate drag... all these things would be obvious though at somepoint on the long flight through a instrument check though.
Since you stress scenarios, but have a screenshot prooving full speed capability, I'm leaning twords you selecting more fuel than the others as the most likely reason. In scenarios the fuel burn rate/arena setting is usualy 1.0 (in the MAs it's 2.0). They rarely take a lot of fuel, often because 50% in a buff will yeild well over 2-hours flight time at full throttle, and air spawns negate the lengthy climbout process. And snapshots or multi-round scenarios which feature at most 1-1.5 hours of flying (and often air starts) per round, it isn't unheard of for buffs to only need 25% fuel.