Is there a number of event sheets that starts causing a performance issue in Construct 2?
What are the other biggest performance issues when using C2 programmes on iOS / Android.
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Short answer is no.
Your main performance issues are fillrate and number of objects with behaviors.
You should read the tutorial titled 'performance tips for mobile devices' and also 'optimization - don't waste your time'.
Optimisation: don't waste your time
The number of event sheets won't affect performance - only the total number of events included on a layout.