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My config is following:

Mac Book Pro
Retina, 15-inch, Mid 2014
Processor  2.8 GHz Intel Core i7
Memory  16 GB 1600 MHz DDR3

Apple SSD Controller:

Vendor:   Apple
Product:    SSD Controller
Physical Interconnect:    PCI
Link Width: x4
Link Speed:   5.0 GT/s
Description:    AHCI Version 1.30 Supported

Command for test:

time dd if=/dev/zero bs=1024k of=1G count=1024

Before enabling FileVault [1.338061 secs]

1024+0 records in
1024+0 records out
1073741824 bytes transferred in 1.338061 secs (802460985 bytes/sec)

real     0m1.342s
user     0m0.002s
sys     0m0.443s

After enabling FileVault [1.008163 secs]

1024+0 records in
1024+0 records out
1073741824 bytes transferred in 1.008163 secs (1065047865 bytes/sec)

real    0m1.011s
user    0m0.002s
sys 0m0.341s

May some optimization was enabled. Anybody knows?

Similar result was reported by Gabi int this question:

Does FileVault encryption slow read/write access on an SSD?

1 Answer 1

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SSD performance is known to degrade over time, if no or poor garbage collection is enabled or not working. The later can occur if TRIM is inactive, the disk is very full or it has no time to do garbage collection. So if the SSD wants to write data, it will need to delete old data first, slowing down the process.

When encrypting the SSD, all its contents gets re-written and free blocks are liberated. This will result in faster write speeds. Reformatting a disk is an effective way of restoring write speeds.

Just a theory however...

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