SMB3 frame drop on NES console?

Hey, I've been replaying Mario 3 a lot more lately but I've been noticing that when there are lots of enemies on the screen and I'm blasting through the level, the game can slow down a little. Is this normal or is there something wrong with my cartridge / system?

It's normal. Tons of NES games have slowdowns when too many enemies are on the screen.

It's safe. For games such as SMB3, this can happen when there are lots of enemies on the screen. Though I never encountered any freezing when I played it many years ago.

Pretty much all NES games do this in certain parts of the game..

In Dragon Warrior games, you'll start seeing crazy flickering and problems when you line up 5 characters along the same line. This includes NPCs. NES had this issue all the time. Even on SMB2.