Daniel is right.
According to my experience,
you need to bump the shutter speed without the fear of ISO, while working with extreme teles.
If the daylight is good then you are lucky, if not accept what you had gotten.
Another important thing is to be as close as possible to subject, I mean very close.
Tele doesnt mean to get excellent shoots from faraway distances.
You can only get snapshoots of far subjects, dont expect to get more, if you dont have 20k$ lens .
Even if you have that spectacular lens, atmospheric conditions always in the game; heat, dust, wind, humidity etc.
And I am also in big trouble with hand held big lens.
But professional tele shooters find 1-2kg weight for the lens fair enough.
These are my a few cents.