BUG: Most levels lag when being played on Android devices

[Alzter]

SuperTux Classic version: v0.4.1
System information: Android Devices Only

Describe the bug
Ever since the level editor was added into the Android release of SuperTux Classic in version v0.4.0, the game now lags when playing most levels.

To Reproduce
Steps to reproduce the behavior:

1. Play practically any level in SuperTux Classic on Android (v0.4.0 or above)
2. The game will lag. The lag ranges from very little to very noticeable depending on the level. The lag tends to vary randomly and come in "spikes" as you play through levels. Sometimes levels are smooth and other times the game noticeably stutters.

Expected behavior
The game shouldn't lag when you play through levels in STC for Android.

Fix strategy
Whatever changes were made in v0.4.0 have clearly made the Android version of the game lag more. v0.4.0 dramatically restructured the game's file structure and scene referencing to support the newly added level editor, so it's possible that whatever changes were made to introduce the level editor have inadvertently made the game more processor intensive and therefore more slow on mobile devices.

The source of the lag will need to be identified and relevant optimisations will need to be made to remove the lag on Android.