Calling `Exponent.Amplitude.logEvent('someEvent')` affect UI performance?

It shouldn’t.
but if you are concerned even slightly about that you could put everything inside of requestIdleCallback

For example:
requestIdleCallback(() => Exponent.Amplitude.whatever)

basically all it does is enqueue an operation to send over bridge so on the js side, close to zero cost.

(h/t @notbrent)