I asked the director of JET one time "if you had a blank cheque, similar circumstance to the manhattan project, when would a fusion power plant be ready".

His answer was that we could just build a massive machine. There are a lot of problems in MCF surrounding stability and confinement time that are so much easier in big machines but big machines are extremely costly. If you had unlimited money then you could build a stupidly expensive machine and eliminate some of the physics and engineering challenges (you do introduce some others, mostly in the diverter region).

As for ITER in particular, it is so close to being built that budget isn't really the issue. Sourcing components and waiting for them to be built is just time consuming, I suspect you could shave some time by expediting some things and increasing construction staff etc.. If you had triple the budget from day 1 then you would probably have a bigger and more complex and powerful machine, you would definitely be able to shove it through some of the commissioning and design and site location phases but I doubt construction is that much faster.