In my opinion, the biggest drawback of the Raspberry Pi is the extremely slow SD bus. This is the result of a single chip computer with everything like the USB and Ethernet all running on the same bus.
The Raspberry Pi 4 greatly improves the SD bus, but SD cards are still insanely slow for random IO and interactive usage. The type of SD card also can greatly impact the performance with only one card performing well.
The Raspberry Pi 3 and 4 can boot directly off SSD and can dramatically improve the performance of your Raspberry Pi. In fact, the performance can be as much as 50 times as fast as an SD card.
Below you will see a comparison of the Samsung EVO+ 32GB SD card, the best SD card you can use for a Raspberry Pi and a Samsung 850 EVO, a high-quality SATA SSD drive.
The first test will be Max Sequential IO and will reflect the maximum performance you can reach on the device. It is important to make sure you are running the SSD drive in UASP mode as this will result in roughly 50% more performance.
SD Max Sequential IO
/dev/mmcblk0:
Timing cached reads: 1774 MB in 2.00 seconds = 886.89 MB/sec
Timing buffered disk reads: 132 MB in 3.01 seconds = 43.90 MB/sec
SSD Max Sequential IO
/dev/sda:
Timing cached reads: 1726 MB in 2.00 seconds = 863.49 MB/sec
Timing buffered disk reads: 1096 MB in 3.00 seconds = 364.74 MB/sec
The SD card comes in at 43.90MB/s while the SSD drive comes in at 364.74 MB/s. This is an 800% improvement over the SD card, keep in mind most SD cards are far slower. On a Raspberry Pi 3+ and lower the performance will be significantly worse as the SD bus is considerably slower.
This is an impressive improvement, but Random IO will be even more impressive and more reflective in the time of load most applications will require.
SD Random IO
Run status group 0 (all jobs):
READ: bw=2636KiB/s (2699kB/s), 2636KiB/s-2636KiB/s (2699kB/s-2699kB/s), io=375MiB (393MB), run=145640-145640msec
WRITE: bw=879KiB/s (900kB/s), 879KiB/s-879KiB/s (900kB/s-900kB/s), io=125MiB (131MB), run=145640-145640msec
Disk stats (read/write):
mmcblk0: ios=94861/31905, merge=817/106, ticks=7571517/1658002, in_queue=398620, util=94.41%
SSD Random IO
Run status group 0 (all jobs):
READ: bw=64.0MiB/s (68.1MB/s), 64.0MiB/s-64.0MiB/s (68.1MB/s-68.1MB/s), io=375MiB (393MB), run=5770-5770msec
WRITE: bw=21.7MiB/s (22.7MB/s), 21.7MiB/s-21.7MiB/s (22.7MB/s-22.7MB/s), io=125MiB (131MB), run=5770-5770msec
Disk stats (read/write):
sda: ios=93754/31258, merge=7/1, ticks=20034/5610, in_queue=23060, util=98.28%
Here you can see the SD card has 27MB/s read performance and 0.9 MB/s write. The SSD drive has 68.1 MB/s read performance and 22.7 MB/s write performance. While the random read performance of the SSD drive is only 250% faster, the random write performance is 2500% faster.
As with most disk devices, the write performance is significantly slower than reading. This is reflected in all of the above tests.
If you are using a Raspberry Pi as a desktop or using it for anything that puts any load on the SD bus, an SSD drive will be a huge performance improvement.
