Slow disk reads on / when booting up

[tight sonnet]

I am using an HDD and when i boot up there is about 1-3 minute wait for applications to start working properly without lagging for 20 seconds. So i decided to do a test of the disk speed on / using hdparm -Tt /dev/sda1

The results were:

 Timing cached reads:   24542 MB in  2.00 seconds = 12285.61 MB/sec
 Timing buffered disk reads:  26 MB in  3.03 seconds =   8.59 MB/sec

Did it again:

 Timing cached reads:   33570 MB in  2.00 seconds = 16811.47 MB/sec
 Timing buffered disk reads:  30 MB in  3.77 seconds =   7.97 MB/sec

Third time:

/dev/sda1:
 Timing cached reads:   31550 MB in  2.00 seconds = 15799.81 MB/sec
 Timing buffered disk reads: 402 MB in  3.01 seconds = 133.66 MB/sec

So it became faster on the third try/more time passed since the boot. How can this be explained? Is this something to worry about? I've been having big latency on the HDD when using MongoDB as well. Sometimes individual writes spikes which go up to one second.

[mellow trout]

Linux kernel caches its IO

You can view how much data is cached from output of free

Note the buff/cache field
e.g. my system has 6.5 GiB of data cached

[tight sonnet]

free -g to get readings in GB

[mellow trout]

It is normal for a desktop system, Linux, Windows or any other operating system to freeze within the first few minutes after booting.

And it is common for applications to take several seconds to load when they are launched for the first time
On a HDD

[tight sonnet]

There are a few services that can optimize it afaik. Like irq-balance, ananicy and preload.

[tight sonnet]

So if i have an ssd that caching process would be faster?

[mellow trout]

Yes, regular SSDs would be much more responsive than most HDDs because
IOPs (input/output operations per second) of SSDs are typically 2-5 orders of magnitude higher than HDDs.