Hi folks, will the performance of a pump decrease prior to the motor going to earth or is there no warning before this will happen?
Any help would be much appreciated.

The sump pump will usually run OK until it trips the breaker.

A GFCI or RCD will trip though.

It is a problem that should be addressed.

The vsd tripped however when testing with a multimeter it did not show any sign of leakage. I was just wondering if the pump would not perform at its best prior to the leakage.

What error did the drive show when it tripped?

The proper tool to test a motor to ground is a megger.

Insulation weakens with age. Period. Stop.

Some insulation is so robust that it can hang tough almost forever: MI.

Weakening insulation within motors is tested by 'megging it' with -- a Megger.

Google and explore.

A known high static voltage is imposed upon the windings, one by one and their RC curves are monitored.

This is how the very beginning of corona discharge is spotted.

Leakage at high voltage precedes leakage/ shorting at operating voltage.

So, there is a way to track a weakening motor before it goes all together.

A regular DMM is not going to spot anything until it's toast.

A GFCI/ RCD will trip before the windings are a fuses mass of metal. Even so, the cure is the same: you need to pull the motor and rewind it or replace it.

These days, motor replacement is almost always preferred.

Being machine built to the latest standards makes them both cheaper than a rewind but cheaper to run, too.

Which is why all of the motor rewind shops of old around here have gone belly up.

Tesla is taking the food out of my mouth. Actually I was motor winder for a while and Tesla has it. If the pump is so rare and the motor so special that an off the shelf replacement cannot be found then rewind makes sense. Or if the motor is actually expensive enough to ecenomically rewind.
So if you think the pump is nearing the end of life then buy a spare and make sure you have a high water alarm.

As for testing the condition of the motor a partial discharge device or maybe some of the new ultrasonic test equipment if a local engineer has one? PD testers are pricy and rare.

A GFCI is a cheap earth leakage tester.