My B280F turns over and start almost immediately when cold.
But when it is warm, it has a hard time starting. I have to keep
the starter turning and forcing throttle, then it will finally go.

Any suggestions?

Mine did that before the major overhaul but was much better afterwards.

Try setting your intake valve clearances a bit loose. If my memory is correct, the loose cold spec is around .004". I went to .006" without a substantial increase in valve clatter, and the car developed much more cranking compression and felt snappier when the weather was hot.

One of the other things I changed was the fuel pressure regulator. A leaking regulator can allow vapour lock when hot but it smartens right up again when cold.

If I had to guess, I'd say your intake valve clearances are the culprit. I'm also guessing it feels rather sluggish and laboured when the weather and engine are hot, but relatively sharp when cold.

I'll try the easy fuel regulator first.

I don't think this motor has ever had its valve adjusted (96K km)
so that adjustment is overdue.

I'd start with the free one first, if it were me. It could be fuel pressure regulator or any of the sensors, so don't just open the parts cannon on it without doing whatever you can do to figure it out.

I don't relish popping the covers, especially the side with the compressor.

I'm also going to Seafoam soak the upper cylinder and valves. Can't hurt.