So for each block, it has to check (up to) thousands of UTXOs, and chainstate tends to grow over time. If it fits in your RAM, it's quite fast. If it doesn't come from RAM, it has to read and write thousands of UTXOs from disk for each block it verifies. Even on SSDs that just takes time, and USB only makes things worse.
For additional information,
1.
https://statoshi.info/d/000000009/unspent-transaction-output-set?orgId=1&from=now-5y&to=now&timezone=browser&refresh=10m shows total and size UTXO growth in last 5 years. Peak UTXO size was about 12.8GB.
2. AFAIK the read/write for those UTXO considered as random read/write operation. So you'll notice the slowdown if you use HDD, very cheap SSD or other slow storage.