Hi, I just made a modpack, and I can't figure something out for the life of me. Whenever I boot into a world, and I try to break a block, it's much slower than usual. It seems to be all blocks, and the hardness of the block does still change how long it takes to break, but it's still much longer than usual. Here's all the mods in my modpack, minus all the support mods that are needed for certain mods to function.
Veinminer and Treecapitator both increase block breaking time but also break all the blocks of the same type, i.e. all logs of a tree, or all iron ore deposits linked together. That might be what you're experiencing.