Create performance_improvement.md

.github/ISSUE_TEMPLATE/performance_improvement.md

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+---
+name: Performance improvement
+about: Suggest a method to improve the performance of this project
+title: ''
+labels: performance
+assignees: ''
+
+---
+
+**What's the issue? Please describe.**
+A clear and concise description of what the problem is. Ex. When doing X, Y happens because [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Tests**
+While an idea might improve performance on paper, in practice it might not. It's really important to test your idea out first!
+
+**Additional context**
+Add any other context or screenshots about the feature request here.