Determining acceleration goals early in the development is necessary because the ratio between the acceleration goal and the baseline performance drives the analysis and decision-making process.
Acceleration goals can be hard or soft. For example, a real-time video application could have the hard requirement to process 60 frames per second. A data science application could have the soft goal to run 10 times faster than an alternative implementation.
Either way, domain expertise is important for setting obtainable and meaningful acceleration goals.