Normal behaviour:

1. Place villager inside minecart
2. Place jobsite block beside villager
3. Villager almost immediately picks up profession

Slow behaviour:

1. Place villager inside minecart
2. Create box around villager to block pathfinding
3. Place jobsite blocks outside of the box
4. Place jobsite block beside villager as before
5. Villager takes much longer to pick up profession

A 10x10 grid of barrels was enough for me to notice the issue with my computer. If you  have a storage system using barrels or a trading hall nearby this issue means it will take a long time to set a villagers profession if you are doing it nearby.

Screenshots taken immediately after placing down the lectern. With no nearby jobsite blocks, the profession updates almost instantly, and with jobsite blocks nearby, you have plenty of time to take a screenshot.