metronome lag

### Bug Summary
Relatively recently it feels like, the metronome seems to frequently cause the program to lag even in an almost empty project, which makes it very hard to use and work with for, say for example, getting the bpm right for drum loops
#### Steps to reproduce
Turn on metronome
#### Expected behavior
it's fine
#### Actual behavior
it weirdly lags sometimes (not as in the metronome is sometimes off beat, it makes the program itself stutter and/or lag for a split moment)
#### Affected LMMS versions
1.3.0-alpha.1.430+g999c10e4b

No source file, test, or entry point is named. Start by reproducing the stutter in an almost empty project with the metronome enabled, then trace the metronome and audio-processing paths involved. Done means the metronome no longer causes program-wide lag and the behavior is covered by a regression test.

Written by the indexing model from the issue text.