This regression is introduced in 10.6 by following commit.
commit 898dcf93
(Cleanup the lock creation)

It removed one important optimization for lock bitmap pre-allocation.
We pre-allocate about 8 byte extra space along with every lock object to
adjust for similar locks on newly created records on the same page by
same transaction. When it is exhausted, a new lock object is created
with similar 8 byte pre-allocation. With this optimization removed we
are left with only 1 byte pre-allocation. When large number of records
are inserted and locked in a single page, we end up creating too many
new locks almost in n^2 order.

Fix-1: Bring back LOCK_PAGE_BITMAP_MARGIN for pre-allocation.

Fix-2: Use the extra space (40 bytes) for bitmap in trx->lock.rec_pool.