Update: 05/09/2019 - Ignore, no longer occurring.
Every other map encountered in the game, including the other arenas, is fine; Tundra Arena, however, consistently immediately grinds to a halt (one frame every several seconds until character death). Unfortunately, this makes the arena little more than an exercise in frustration as there appears to be no way to avoid that stage other than chance.
Decreasing graphical quality to low/very low and disabling checkbox options has little or no impact.
Update - 05/03/2019 - Attempting to reproduce. Has not yet occurred in 0.7.0b.