All versions of the SDK are very lightweight (Android ~140KB, iOS ~3.4MB), and usually occupy less than 1% of apps' sizes. Therefore the SDK effect on the overall time of app download is marginal.
The AppsFlyer SDK APIs are mainly setter/getter methods, or fast asynchronous web services methods to send or receive data from AppsFlyer's servers. Therefore, the performance of the SDK APIs is totally negligible.
In short, using AppsFlyer's SDK has no effect on your apps' user experience.