We noticed regressions and inconsistencies after migrating from 0.18.1 to 0.19.0

Hello folks,

we are trying to upgrade an existing project to `splinter==0.19.0` but we are running into a lot of issues.

We are aware of the deprecations you introduced (the warning was quite useful) so we migrated all the existing code like this:
```python
context.scenario_browser.is_element_visible_by_xpath("//a[contains(text(), 'Log In')]", 30)
```
to this:
```python
context.scenario_browser.find_by_xpath("//a[contains(text(), 'Log In')]").is_visible(30)
```
and the weird thing is that some checks still work as they were working with `0.18.1` while some others give us a mix of these issues:
- the same document which can be parsed with `0.18.1` now with `0.19.0` needs a `time.sleep(1)` (or even longer) otherwise the document isn't ready
- it seems that the `wait_time` parameter is completely ignored: if we use a `breakpoint()` and debug the code step by step, everything works fine because by running it step by step we manually introduce slowness and by the time we run the next line of code, the element is ready to be parsed.

Considering that we haven't done any other change to our code (the initial PR was made by Dependabot which just bumped the dependency number and I only changed the syntax by following the warning message) we can't really understand why the same code doesn't work anymore.

Are you aware of any similar regression or do you know if there is something we should do to make sure the document is fully loaded before we try to access any of its elements?

Thank you so much.