This is a longstanding lag issue with the Nintendo switch Bedrock version of Minecraft.  It predates the recent switch lag issues after 1.18.3.  Whenever magma cubes are visible in the nether (regardless of biome) the game lags significantly.  I have a nether hub at y=15 consisting of the tunnels excavated by TNT with a railroad line on a single row of blocks that runs down the center.  Though pretty well spawn proofed I will get the occasional magma cubes spawning in the tunnels.  As soon as the magma cubes become viable the console lags to the point of complete stops (all actions) for up to 30 seconds or so.  It only seems to be with magma cubes as other mobs do not appear to cause any lag.