Is this write speed normal or I'm I having a faulty disk?

Filevault will slow down the write speed on your tests. I don't normally test SSD speeds while booted from the SSD being tested since the results are not that reliable (Filevault, Spotlight indexing, software updates, etc. all interfere), but I have seen other users show significant differences between read & write speeds on their test results.

@Grant and I both believe your SSD is operating normally. Enjoy the laptop.

Writes are normally quite a bit slower. Adding a write buffer can make the drive appear to write faster, but there is a trade-off.

Some manufacturers choose to add a larger buffer for write data, others use a smaller one. The larger the buffer, the higher the risk that data might be acknowledged, but a problem or power failure may intervene that results in data loss, so this is a real trade-off, not just a cheap-out kind of a choice.

It looks reasonable to me. Writes will always be slower than reads since the SSD has to do more work and preparation when writing to the drive.

Here is a test of my 3.5-in rotating Magnetic drive, installed in an internal bay in a Mac Pro silver tower. This drive was selected because it is especially fast for a rotating magnetic drive.

I think you have NOTHING to worry about.