[Astar] Collator syncing phase very slow ~3-5 bps at 1M blocks height

**Description**

Astar collator syncing phase very slow ~3-5 bps at 1M blocks height. It starts with 600-700 bps, then 20 bps at 700k blocks and then ~3-5 bps at 1M blocks height

**Steps to Reproduce**

Sync from scratch a collator using latest version (the same issue in the last 3-4 versions, I tried them).
I use the same flags as in the official Astar documentation