I changed from a 5 year-old 2TB external HDD to a recent 4TB external HDD and I think the problem is still there. I haven't experimented much today... I have already spent ~ 3 mBTC in the past two days in bitcoin fees 
You can use testnet and save your bitcoin, but you'll spend some time for setup
I ran "iotop -a" and waited for the numbers to stabilize. The result is:
https://i.imgur.com/vJu9ZTP.png
https://i.imgur.com/vJu9ZTP.png
Looks like my guess isn't right since it's far from 100%. Sorry, but i can't anything else that might causes the problem.
I realised that "iostat" gives the average I/O stats and not the instantaneous ones. I moved a large file from the external HDD into my laptop's SSD and saw that it averaged around 30 MB/s
If the problem is I/O speed from the drive, I am not worried because I will launch my app on AWS with an SSD... I am more concerned about excluding other causes.
Code:
avg-cpu: %user %nice %system %iowait %steal %idle
4.3% 0.0% 3.2% 24.5% 0.0% 68.1%
Device tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sde
293.00 36.0M 0.0k 36.0M 0.0k
4.3% 0.0% 3.2% 24.5% 0.0% 68.1%
Device tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sde
293.00 36.0M 0.0k 36.0M 0.0k
If the problem is I/O speed from the drive, I am not worried because I will launch my app on AWS with an SSD... I am more concerned about excluding other causes.
Based on average read speed, do you use USB 2 (whether the port is USB 2 or the external HDD uses USB 2) ?