Does speaking faster improve my fluency and coherence score?

No, and it often works the other way. The published wording describes fluency as speaking "at length without noticeable effort" and coherence as development that keeps its thread; neither depends on speed. A faster delivery compresses the pauses but also compresses the repairs, so a listener hears more restarting per minute. In the recording check above, most candidates who slow down by roughly a tenth find that their repairs become audible as repairs rather than as breakdowns, which is the direction this criterion rewards.

IELTS Speaking Band Descriptors: What Fluency and Coherence Rewards →