Add 12.10 performance improvement issues

/label release post release post item Technical Writing

Engineer(s): @gitlab-com/backend-managers and @gitlab-org/frontend/frontend-managers | Tech Writer: @rdickenson

Add performance improvements to the 12.10 release post.

Includes improvements from

• Issues closed/in workflowverification in 12.10
• Merge Requests merged in 12.10

Key dates

• By the 15th: EMs should draft/submit all performance improvements
• By the 16th: TW lead reviews and applies the ready label.
• By the 17th: Release post manager merges.