Why is the synchronization so slow?

Because of the SPAM, your synchronization time is 100 times slower.

Zcash used to offer a fix fee for all shielded transactions regardless of their sizes: 0.00001 ZEC. That’s very cheap. One day, someone started making transactions with 1000s of inputs/outputs for next to nothing.

Thus, pretty much all the blocks got filled with garbage (most likely, it’s shielded - we do not know what is inside).

Eventually, fees were raised but that took one year, during which the SPAM went uncontrolled.

The SPAM lasted from block height 1 700 000 to 2 200 000 (roughly).

If you have to scan over that period, you will notice that the speed goes down to 1% of what it used to, as blocks are 100 times larger.

The good news is:

1. YWallet can still scan over this period, but it is SLOW. Every other wallet stopped working.
2. You can activate the spam filter. It will not try to process the spam transactions but they still have to be downloaded. That’s GB of data.
3. The spam has ended. Fees are dynamic and the cost of large transactions are higher, making spamming cost prohibitive.

The bad news is:

1. As of Nov 23, only YWallet has been updated to compute the fees according to the new rules. The other wallets can make transactions that are considered spam by the network.