The Famicom Disk System always had loading times for its games. In fact, the Legend of Zelda was released as a launch title and used both sides of the disk. The game required the user to flip the disk at a certain point and would load new data at certain times. Over time, the loads and swaps became more frequent. When you consider the major advantages of the FDS, namely the sound channel and no need for passwords, consider the tradeoffs over their cartridge releases in the form of disk swaps and load times. Whether running a game off real disks, an disk emulator like FDSLOADR or the FDSStick or a NES emulator (Nestopia probably has the best FDS sound emulation), you will have to face these inconveniences. Below let me give some examples of games that were ported to the NES and their loading and swapping requirements.
Zelda no Densetsu - The Hyrule Fantasy
Yume Koujou! Doki Doki Panic
These titles are the easiest to deal with when it comes to swaps and loads. Both require you to swap to side B at the title screen. Zelda will load each Underworld dungeon level as you enter it and the Overworld when you exit. DDP will load each time you start a new world (but not a new level) and the incomplete ending. Zelda saves to side B, but DDP saves to side A, requiring a swap when you want to save your game. DDP also requires a swap to side A when you want to see the real ending.
Palutena no Kagami
Metroid and Palutena require you to swap to side B after you start a game. Metroid will load each of the five areas as you go up or down the elevators. It will require you to swap back to side A for the ending and for saving. Because Metroid saves to both sides of the disk, the swap instances take upwards of a minute.
Palutena has short loads before and after every boss palace level. When entering Medusa's level, 4-1, you will have to swap to side A. There is a final load after you defeat Medusa.
In all versions of this game, there is a load before every level. The last level where Dracula is the boss has a second load between stages 17 and 18. However, different versions of this game handle their swaps at different points in the game. The same saves to side A.
In v1.0 and v1.1 of the game, you will swap to side B when you start the second level. You swap back to side A after you beat Dracula to view the ending. In v1.2, you will swap to side B when you start the first level. When you reach stage 18 in v1.2, you will swap back to side A. I believe that Konami was trying to give more experienced players a break in v1.2 by not requiring them to swap when they try to beat Dracula for the umpteenth time.
Legend of Zelda 2 - Link no Bouken
Dracula II: Noroi no Fuuin
These games win the prize for the most loads of any that were ported to the NES. While both require you to swap to side B to start a game, they have many more loads. The Legend of Zelda 2 has a load for each entrance to and exit from the eight towns and seven palaces. Also, there is a load when you cross a land boundary. There are four land boundaries in the game: Western Hyrule, Spectacle Rock, Eastern Hyrule and Maze Island. Finally, you must swap the disk to enter the Great Palace (the last palace) and there is a load for the ending but the disk swap does not occur until the credits roll. The game mercifully lets you continue at the Great Palace if you lose all your lives (but not at previous Palaces), so you don't have to swap the disks again until you play a game from the main menu.
Similarly, Dracula II: Noroi no Fuuin has a load for every time you enter or exit one of the seven towns or the screen with one of the five Mansion entrances. Even when you just want to walk on past the Mansion entrance, it is annoying because you have to put up with two loads! If you get knocked back from a screen, you have to suffer through two loads. If that was not bad enough, in the last half of the game there are frequent loads and swaps. To give you an exact description of the loads and swaps, here is a plan of the last part of the game and the areas you must visit to complete the game in an efficient manner :
Town of Ondol - > Deborah Cliff - > Bodley Mansion - > Uta Lower Road 1 & 2 - > Debious Woods - > Joma Marsh 1 - > Laruba Mansion -> Joma Marsh 1 -> Debious Woods -> Uta Lower Road 1 & 2 -> Bodley Mansion -> Wicked Ditch -> Town of Doina -> North Bridge -> Dora Woods -> Town of Yomi -> Vrad Graveyard -> West Bridge -> Castlevania -> Ending -> Title Screen
-> Loading and Swap to Side A
-> Loading and Swap to Side B
Before this sequence, you had only one swap to side B when you started a game. There are nine loads and five disk swaps in a somewhat short period of the game map! No wonder why even Japanese gamers appear to rate this game substantially lower than its predecessor or its sequel!