2B joins Shirou in the Holy Grail War, her role is to survive until the end.
2B has her Pod with all its abilities, her four weapons Virtuous Contract, Virtuous Treaty, Virtuous Dignity, and Virtuous Grief, and is equipped with her most powerful Plug-In Chips.
Scenarios
Fate route
Unlimited Blade Works route
Heaven’s Feel route
All scenarios have a speed equal and unequal parts to them.
Shit, without Avalon, I can see Shirou not really making it through the Fate Route and even then, I don't see 2B even remotely defeating Gilgamesh in any capacity.
UBW is a little better since she wouldn't really be fighting anyone major like him but it will still lead into having problems with Medea since Magic Resistance A wouldn't be with 2B.
HF would have her fall into the same issues and die even faster against Shirou and Medusa so(Hell, I don't see her defeating Heracles either)...
Oh okay, then I think she would do a little better but it's hard to say if she can defeat anyone in that Grail War without some level of trickery or taking advantage of flaws
It would slightly help against a decent number of Servants there but not all of them.
It would really only help against Cursed Arm and Medea since they aren't combat capable.
Servants can explicitly react in low microsecond-high nanosecond level speeds and shown to react to FTL attacks so the reflex boost has to be significant and slowing down time will only help a bit since she really needs to deal with Artoria, Gilgamesh and Heracles the hardest and they aren't going down in 1-2 hits AND their reactions and danger senses are absurd for Servants.
NieR character has Beepy, the P-33 model that can process time in nanoseconds and is several thousands of years out of date. A2, 9S, and 2B can react and dodge against the Flight Unit's beam guns and beam missiles, bullets and missiles propelled by using particle beams. 9S react to an EMP blast.
She does have intangibility that would help her avoid some attacks or her nanomachines that would help her regenerate (there are Plug-In Chips to increase the speed of her regeneration, and her Pod could help her with his healing and generate forcefields), hopefully enough for Shirou and his allies to finish off high-level Servants in those scenarios. Her Pod could teleport her. Plug-In Chips could make her invulnerable for 6 seconds after taking damage and absorb or reflect damage.
Her Pod has some hax abilities like gravity manipulation and generating pseudo-black holes.