IRF530 getting very hot

I have an IRF530 connected to a 5V micro. The trace is on a PCB and very short, with only a pull down resistor on the gate:

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Here is IRF530 Datasheet

The MOSFET is switching a long length of 24V LED strips, which draws 6A. There is a 5m 2 core cable from the MOSFET to the strip.

When I set the pin to high, the mosfet gets very hot. On shorter lengths of LEDs, it only gets marginally warm. When I PWM the mosfet (f=500Hz), it still gets hot but takes a little longer to warm up.

How come this is happening? I am above the threshold voltage of 4V, so I assume the FET is not in a linear state, and the power dissipation according to Rds(on) is only 0.96W, which the un-heatsinked (un-heatsunk?) package should handle.