GOING TOO SLOW — The most obvious problems with going too slow for singers or wind instruments is SUSTAINING LONG HELD NOTES or carrying over a PHRASE. The next most obvious problem is the tendency to go FLAT, (or LOSE PITCH) because of lacking breath support. The right TEMPO usually helps with being able to sustain pitches and stay in tune. (Pianists don’t have to breathe every few beats to sustain a note, so they sometimes don't understand the tempo problem.)
PRACTICAL EXAMPLES: In Primary, unfortunately, some accompanists do not have much training or skill. There always seems to be that lady at the piano who can play in just one TEMPO. And whether that TEMPO is FAST or SLOW, it is still not right for every song.
Singing a song like “I’m Trying to Be Like Jesus” too quickly turns it into a galloping horseback-ride and doesn’t maintain a reverent attitude. Singing “Love One Another” too slowly makes it sound like a funeral dirge and the long phrases become impossible to sing.
The proper TEMPO really does matter. Always try to practice at the METRONOME MARKING. I promise, composers do try to think through what would be best for their pieces.