Changing or reloading Minecraft's texture pack, in any situation, causes an absurd delay where the client is entirely unresponsive. This applies to:

• Changing texturepacks via the main menu, under Options
• Changing texturepacks ingame via the menu
• Server textures activating when you join a server
• Server textures deactivating when you leave a server
• Refreshing textures when pressing F3+T

The delay ranges anywhere between 7 and 15 seconds for me. The lower end of that range (viz. around 7 seconds) appears when activating a texturepack, even if it only modifies a single texture. The delay tends to be almost twice as long (viz. up to 15 seconds) when switching back to the default texture pack.

This is disruptive, if not altogether deadly, when joining a server with custom textures: the client is unresponsive for at least 7 seconds, during which time any number of bad things can happen (especially if you logged off in the middle of danger).

I should note that, in the past, texturepack changes have not caused client freezing in excess of 3 seconds for me (often as little as about 1.5 seconds), which is understandable.