Ok my iclone is running at 60 FPS
I have 1 avatar, no other polygons. 45000 polygons total.
Ive got about 100 separate keyframes tied to this avatar, and now when I turn the Limb Gizmos dials on any frame, its very sluggish.

Is this normal and expected?

I dont understand why its running at 60 FPS, but the joint gizmos are sluggish to move.
If I add a primitive shape to the mix, the gizmos for this run smooth.
If I add another complex avatar, that has no keyframes, the gizmos for that avatar run smooth.
If I delete most of the keyframes for my first avatar, the gizmos run smooth again. 

Am I to believe that if I am manipulating an avatar for a certain frame , then the program is calling each keyframe (hence the slow down) and determining how its effected by my
manipulation of that single frame.  surely the program only needs to call the previous keyframe and the after (if any) .

Am I also to believe that im better to have the avatar at the half way point be replaced by the same avatar (using visible ) so its gizmos work faster. 

--- Laptop Specs ---
i7-8750 Octocore
16Gb Corsair Dominator
8Gb 1070 GTX

Iclone can get sluggish if there are many many keyframes, dont know the exact range at which it could cause this,that is kind of normal and expected BUT what is weird is that other avatars dont have this lag and like you said it is weird that it runs at 60 fps. When you say drop a primitive in the mix, do you mean you just place it the scene? 

One thing you can try, be sure to hit save as before doing this, is to either use collect clip and save the motion clip and then place that clip instead of your animation or right click on the motion clip that has all your keyframes and click flatten, then see if it you can animate without it being laggy.

Is your avatar special in anyway? Is it a cc avatar?