Jira 8 and 9 ship with a default value of 4000 for the "Reindex batch".

For the default 20 Index Threads, this means every Thread would iterate over the batch 4×, if they'd all complete at the same time. (20 Threads × 50 Issues pulled each iteration = 1,000)

Jira Full Reindex would be quicker if this batch was way bigger. Customers have reported considerable Full Reindex speed improvements when configuring batches of 40,000 or 80,000 and even higher.

jira.index.background.batch.size = 40000
jira.index.issue.maxqueuesize = 40000
jira.index.sharedentity.maxqueuesize = 40000

This is specially important for customers that increase the Index Thread pool.

If a customer bumps the Index Threads to 80, this means they'll only run one iteration every batch, as 80 × 50 = 4,000. As the JiraTask Thread waits for the batch to be depleted and all Index Threads to have finished their last batch before reloading the "batch of batches", this downgrades performance very quickly. There will likely be more waiting time than running time overall in the Reindex process.
Going above 80 Threads wouldn't make effect, as Jira may not even spin up the other Threads as there just aren't enough Issues in the batch to be distributed among all Threads (keeping the default 50 Issues per iteration for each Thread).