Very slow to load blog page in admin with a lot of categories and tags

[MarcioBarrientos]

I have a blog page with 16,000 blogposts, 300 categories and 2,500 tags and it have a poor performance loading the blog page in the admin.

I detected the problem in the GridField component "GridFieldMergeAction" in "GridFieldCategorisationConfig" class, I just disabled it and now it loads the blog page in a couple of seconds, I never had used that function and I want to know if it is ok to send a PR with a setting in the blog page to enable/disable that component.

I know this site have too many categories and tags but I cant just delete them.

[dhensby]

I don't think a PR to disable a feature that becomes a problem with incredibly large numbers of categories/tags makes sense.

It's probably better just to note this in the docs (performance degradation when there are large numbers of posts/categories/tags) and potential ways to mitigate any issues