Apple - What are the performance impacts of blocking a contact with 10,000 numbers in it?

There's no impact in performance. TrueCaller uses CallKit, the iOS provided method of blocking spam calls, and behind the scenes they are all loaded into one contact.

The only issue would be if you wanted to manage that contact manually rather than using an app that is designed to do it. Swiping to delete hundreds of contacts is a pain!

Anyway, there's an app called WideProtect that uses the CallKit to do the blocking and will let you set ranges of numbers to block.