General purpose SSD ('gp2') volume type is just slighly more expensive than
Magnetic ('standard' / default in AWS), while the performance gain is pretty
significant.

So far, the volumes were created only during testing, where the extra cost
won't make any difference. In future, we plan to introduce QoS classes, where
users could choose SSD/Magnetic depending on their use cases.

'gp2' is just the default volume type for (hopefuly) short period before these
QoS classes are implemented.