Execute command in repeating command block causes extreme lag

Rooted Samsung Galaxy Tab 4.0
Ive been trying to use the execute command in a repeating command block, but whenever I use it it just crashes or lags my game to the point that it has a 10 second delay. Such as this command

// /execute @a ~ ~-1 ~ teleport @e[type=silverfish] ~ ~ ~
 

It also lags if you use a impulse command block with a redstone clock, but only lags when the command block is activated. Its either the command itself or how the command updates the used entities.