Amazon RDS MySQL - write throughput limited to 15 MB / s

We are duplicating the data inside a rather Amazon RDS MySQL big table, about 2 GB of data. We are doing that by having batches of 30 entries moved at a time (INSERT INTO etc SELECT FROM ... LIMIT x OFFSET y). The script is working correctly. However, on my local machine with similar anonymized data took about 4 minutes. In RDS it seems to be taking hours. When investigating seems that RAM and processor are not under stress. However, the write seems to be limited at 15 MB / s. The volume is a gp2, that should have at least 128 MB / s. What else can cap that speed?