My Little Pony....IN SPAAAAACE
Feat happens in My Little Pony episode 67 "Flight to Cloud Castle" (part 1).
Garth dreams about what he can do with the power of true love, which includes "travel to the stars above". At the end of the song, Garth, while riding Heartthrob, to fly past what seems to be a nebula.
Feat here, starts at around the 0:27 mark
The nearest Nebula is the Helix Nebula, which is about 215 parsecs away from Earth. The whole thing seems to take place in at most an hour, so we'll use that as our timeframe.
215 parsecs / 1 hour = 1.84283522 × 10^15 m/s or 6,147,036c
This is the third FTL MLP feat done in a dream/imagine sequence so far. The best speed feat I've found that actually counts is Whizzer flying to Sqreerah and Firefly flying to Megan's house, both of which can't be calculated since G1 geography is random.