Restricted or 'plugged' converters are easy to test for by taking exhaust backpressure readings. Since few people have the equipment for testing backpressure, I suggest that you loosen the exhaust in front of the converter (usually easiest at the manifold) to create another path for spent gas to exit. A short test drive would then confirm or disprove exhaust restriction.

Even more important than verifying a plugged converter is discovering why the converter plugged up. If the converter melted internally due to a too-rich fuel mixture, the replacement will suffer the same fate if the cause is not corrected.

The most noticeable symptom of a plugged converter is lack of engine performance. The motor may start and idle normally, but will struggle to accelerate when the throttle is opened. It can feel very much like brakes dragging badly. If the restriction is bad enough, an engine can be held at WOT with no load, and it will struggle to gain rpm.

The worst I ever saw was an older Subaru owned by a college student. It was very difficult to start, and literally would not go over 15 mph or 1000 rpm. The owners complaint? It took too long to 'warm up'...refering to the length of time necessary for the engine to remain running and able to move the car without stalling.

__________________
1989 300 SEL that mostly works, but needs TLC