Now that 512x packs work, I tried to use one but I experienced massive lags while 256x packs don't cause any lag. I allocated 8GB of RAM to Java and Minecraft.
Btw Minecraft uses the full 8.406.560 KB of RAM when using this texturepack.
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I am marking this as a duplicate of MC-2176, but it seems to me your issue is hardware. As with any game, just because you can increase graphics settings doesn't guarantee that your system will properly support it.
Guys I have a new pc with 8x3605 MHz processors and 16GB of ram of which 8GB are allocated to Java. I doubt it's my pc as it can handle ANY other game without lags on ultra settings.
I know that 512x packs have to take a lot of ram but I have enough resources to give.
EDIT: Works just fine with 14 GB allocated (uses 10). I'll stick to 256x packs tho as there is not much visible difference.
512-packs are 4-times bigger than 256-packs and therefore need 4 times as much RAM.
Also you need a better CPU and/or graphics card for 512-packs to work.
It's not Minecraft's fault, it's your computer.