Now here is the way how you test your memory speed
Download from play store and install, open the software you want to test the memory card or your built-in memory space, it will come out the result after run out the test.
In my hand, the Zenfone 3 Deluxe 6GB RAM 256GB ROM test results built-in 256G space reading speed is up to 508.89 MBps, writing speed also have 264.69. MBps and I consider that is quite good, but the 128GB UHS-I memory card is only 56.93 MBps reading speed, as to writing speed is only 18.06MBps, this will be fulfill most of the users but if you looking for recording 4K images than it may be a bit difficult.
Want to know how fast your phone's memory speed are? quickly to download and install it!
Here is that link
Feel free to share your memory speed with specific spec