Volto 3 is not designed for high-draw pedals. So what is considered to be high-draw?

This is a good question.  There is a little bit of an art to this but determining what pedals are compatible with your Volto 3 comes down to three main considerations:

1. TOTAL POWER DRAW - The first consideration is the total power draw. Some digital pedals exceed Volto's mA output (like Line 6 HX Stomp).
2. CONSTANT DRAW PEDALS - The second consideration is that some digital pedals, for example, a Strymon Timeline, draw the battery when activated and in use, but also when off - in ready mode. These types of pedals reduce Volto's battery charge at a fast rate and do not maximize the length of charge life. Yes, they can be powered by Volto, but they use a lot of charge.
3. ISOLATED GROUND - The third consideration is digital pedals often require an isolated ground for optimal performance.  Because Volto does not offer isolation, these pedals are best served with a Pedaltrain Spark.

With these considerations in mind let's look at the Strymon Timeline as an example pedal.  We've established that Timeline can be powered by Volto, albeit imperfectly as a result of the constant draw, but depending on the configuration of your pedals, a noise artifact may be produced because the Timeline needs an isolated ground for perfect operation.

The general rule is - if the pedal can run on batteries, Volto is likely a good choice.  If it does not, you have to weigh your needs against the considerations above.