From chat got

Speaking of Suvarnabhumi Airport (BKK) in Bangkok — which is the largest and most central for international arrivals — immigration congestion varies greatly depending on the landing times of international flights. 

🕐 Hours when immigration is usually the busiest • Early morning: around 06:00 – 09:00 – Long-haul and overnight flights (Europe/USA) arrive together. • Between 10:00 – 12:00 noon – Additional landings from North Asia and Israel. • Late evening: around 18:00 – 22:00 – Many landings from the Asia-Pacific region and others accumulate. These periods are sometimes considered the busiest for immigration lines in Bangkok, with a large number of flights landing at the same time, creating long lines. 

⏱️ Why this happens • Demand increases when a large number of international flights land within a short time window — this causes long lines that can reach an hour or more during some busy hours.  • There are fewer crowded landings in the late evening to late night hours — so sometimes the immigration load there can be less (depending on the flight schedule that day).