PostgreSQL COPY: is it always faster to delete and recreate indexes?
My guess would be going with some benchmarking, measuring the time it's taking you to recreate the full index, the time it takes to insert 10k rows without index, with index. then I'd try with 1000 rows, 5000 rows, 10000 rows, to try to figure out a trend.

Because even if you try to reduce the amount of factors that could affect the operation time (as Auto-Vacuum) it still depends on your hardware and your whole postgres.conf

BONUS: if you share with us some benchmarks, we could gladly help you to figure out the trend.