We can test server disk read / write speed from SSH. Below command will show the speed that the server can read and write a 1GB file.
Login to your server via SSH and run below command:
dd if=/dev/zero of=sb-io-test bs=1M count=1k conv=fdatasync
Higher scores is better.
SSD disk will give you very fast read / write speed compared to SATA disks. There are many providers offering SSD VPS like DigitalOcean, RamNode etc