Test Case Execution Time Slow Issue:

Issues:
Test case execution time is aggrandizing continuously. For this reason, it needs to do something new.

Solution:
Kill your browser, after every test case execution.
Sleep time decreasing.
Sleep time decreasing is not always helpful. So, you can kill your browser and make a new call in every test case execution.

Kill your browser in @afterMethod as like below:
    @AfterMethod()
    public void methodClose() throws Exception {
        setDriverAppiumValNull();
    }
    protected AppiumDriver driverAppium;

      public void setDriverAppiumValNull() {
        if(driverAppium != null) {
            driverAppium.quit();
            driverAppium = null;
        }
    }

“if(driverAppium != null)” - This condition checking helps you to NullPointerException.