I rarely use Appium nowadays because I'm now at the managerial level, but the last time I used it, whenever I selected and clicked on an element, Appium was very slow. I tried to debug it, but I still couldn't find the problem, so this is an area for improvement in the solution.
Another area for improvement lies with the connector and server. For example, the effort to get into the local machine sometimes causes the emulator to become slow, which then leads to failure in testing, and this is the usual issue I've encountered from Appium.
An additional feature I'd like added to Appium in its next release is being able to do automation in iOS without using XPath and the name of the element. In Xcode, you can use previous UI tests for detecting elements, but in Appium, you have to use Xpath and the element name instead of being able to directly put the X-UiPath, which is what you can do in Xcode. In iOS as well, sometimes the element doesn't have a name or a path. Sometimes, there's also no element.