Improve dropdown rendering speed

[lekoala]

Fixes #10174

[michalkleiner]

Failing tests are related, please review.

Apart from that, I'm not personally sure about this change. I get that preparing the list in PHP might be faster, but with that (at a stretch) we can scrape the whole templating system and do everything in PHP.

[lekoala]

@michalkleiner yes i'm not sure why the tests are failing i need to review this and see what are the tests doing

the template system is fine for simple things, but not great for loops. rendering large list of items (eg: a dropdown with country names) is quite slow. i do think there is an issue with the templating system itself, but that's clearly out of scope :-)

[michalkleiner]

Saw the comment here so I got more context now. Let's see what other @silverstripe/core-team thinks about the topic.

[lekoala]

@michalkleiner tests all green ;-)

[GuySartorelli]

I don't like this change. If there's a problem with the templating system that makes it too slow for this and similar scenarios, then resolving that problem is the fix. This feels like a project-level workaround.

I especially agree with Michal's comment: "I get that preparing the list in PHP might be faster, but with that (at a stretch) we can scrape the whole templating system and do everything in PHP."
If we start pulling template logic into PHP here, where does that end? This doesn't feel like the correct approach to the problem. I'm going to close this PR on that basis.

[lekoala]

@GuySartorelli i agree with you: i don't like this change. but when a page takes 2 times more to render because of a couple of dropdowns, that's not something that can be ignored either :-)

ideally, i think profiling should be done on the templating system and see where are the bottlenecks