Steam update durations depend on compression + CPU performance + SSD I/O. Things will be harder when the disk is almost full and live defragmentation kicks in to get free space for contiguous files. Some SSDs are fast enough to keep up with such a load, but a lot of them will quickly hit their DRAM limits and suddenly that advertised gigabyte per second write speed isn't all that fast. Bonus points for when your SSD doesn't have a heatsink and moving air over it, making the controller throttle hard.

Patching 150GiB with a compressed 15GiB download just takes a lot of I/O. The alternative is downloading a fresh copy of the 150GiB install file, but those playing on DSL will probably let their SSD whizz a few minutes longer than spend another day downloading updates.

If your SSD is slower than your internet capacity, deleting install files and re-downloading the entire game will probably save you some time.