Vader should be faster than r2d2 and bb8.
Vader base speed 121 (+ speed from ability 16 Palpatine 16 Vader 8 thrawn 8 Nihilus =48 extra +extra speed from opposition Kenobi and or r2d2 but not counted here) 169 (185)
Bb8 base speed 162 nd r2d2 is 157
Vader should be faster.
Go first force crush culling blade, Palpatine's zeta will give you 5% Turn meter for each debuff culling blade expired which will mean Palpatine and thrawn should be ahead of most opposition.
Edit I forgot about bb8 roll with punches, yeah you are right you need to have better speed mods then they do or drop Kenobi for an extra 8 speed for Vader from a Sith or empire ally.