Flying=Lag

I was playing Minecraft and flying around my world working on projects, and something I noticed was that whenever I would fly or walk on a slab with air under it, or on a multiple-story house, it would lag.  When I say lag, I would drop into 5 FPS.  Then, I get on something solid like dirt on the ground, and the FPS shoots back up to 60.  I would say this is a PC issue, but it does it on my phone too.