Why is NAND flash slower than NOR when it comes to reading?
The difference in read speeds between NOR (few nanoseconds) and NAND (microseconds) is due to the difference in architecture of read logic. just consider the read operation of just one bit (the arrangement of bit and word lines in NOR vs. NAND is a different topic). The read of each memory cell is done by applying convenient voltages to its terminals and measuring the current that flows into the cell. NOR and NAND
memories measure this current in different ways:

NOR:

The read operation is done differentially. The desired cell is biased and at the same time a 'reference cell' is biased with the same voltage. the current in both is measured and then compared. The biasing (both measuring and reference cell), equalizing, and current comparison takes around nanoseconds in modern flash memories.

NAND

In NAND the cells are connected in series and therefore the current to be read/sense is 200–300 nA. This makes the differential sense method difficult; instead, charge integration is used. The bitline is charged by a defined amount and then checked whether it gets discharged (indicating that bit is erased) or not (no discharge means the cell/bit is not sinking and therefore set). The setup time to precharge the bitline (2-6 µs), then to let it discharge and evaluation (5-10 µs), is what takes time and makes the NAND read operation slow.