Add github issue template for performance improvement

[abitmore]

Mentioned by @pmconrad in #1079 (comment):

General remark: I think we need a "Performance" epic - #982 belongs into the same category.

[pmconrad]

I don't think that's very useful.
I think issue templates are mostly meant to help external contributors produce meaningful issues, by giving them a guideline how it should be structured and what information is relevant.
External contributors are unlikely to contribute code optimizations though.

The "epic" that I mentionend is a kind of "meta-issue" that serves a collection of similarly themed but more specific issues.

[cogutvalera]

I agree with @pmconrad ! really useless issue and we don't need to waste time and efforts for it imho ! we should concentrate on more important issues !

[oxarbitrage]

Yea, well, most of the performance issues are added by @abitmore and he probably don't need a template.

I will like his word on this, I don't mind closing the pull request unmerged if we close the issue alltogether.

[abitmore]

OK, I agree to close it.