Cheap SSR's have higher resistances than good quality MOSFET's for controlling the current due to using cheaper parts. People also underestimate the impact of a little bit of extra resistance when it comes to heatbeds & high current applications. Every 1/10th of an ohm can have a significant impact in heat bed heating times, and cheap multimeters are't good for measuring values that lot unless you have good quality leads and can zero the meter properly.
To use overly simplified figures, if you're drawing 10A and add even 0.1 ohms due to cheap parts, you're going to lose a full 1V! Or to look it it from the currents point of view at 12V source, going from 1.2ohms to 1.3 ohms, means current drops from 10A to 9.2A ... which has a big impact on the heat generated.
To use overly simplified figures, if you're drawing 10A and add even 0.1 ohms due to cheap parts, you're going to lose a full 1V! Or to look it it from the currents point of view at 12V source, going from 1.2ohms to 1.3 ohms, means current drops from 10A to 9.2A ... which has a big impact on the heat generated.