How high is your tank from your pump? If it is around 1.5m, the flowrate will only be around 60% of the rated flowrate. If you are using an Atman pump e.g. AT-104 where the max head is only 2.0m, then the drop in flow rate will be even larger.
If you are using a Resun Chiller, then you should be aiming for a flowrate of at least 2,000-3,000L/hr which means you will have to use a pump rated at 3,000-4,500L/hr flowrate and has a max head of at least 3.0m.
If your temp setting is 27 degC, it shouldn't take more than 2 hr to cool down from 30 to 27degC. Subsequently from 28 to 27degC, the time taken should only be around 30-45 mins or so at the max.