I am having similar issues with MindCrack v8.0.1.
Chunk loading seems slower overall, but in certain parts of the world where there is a concentration of mod blocks, chunks load so slowly you can fly over them before they're rendered. I have also observed that walking over the chunk causes it to render. If I stand in one place long enough, I can see a new chunk render every few seconds, but occasionally some of them never seem to show up.
During this time, the client FPS is not noticeably lower. Nor is CPU or RAM usage on the client or server. Having Optifine installed or uninstalled makes no difference. Texture pack or no texture pack makes no difference. Number of players on the server makes no difference.
I suspect it may have to do with Forge 6.6.0, whether or not Forge is causing it directly.
As a test, I upgraded my MindCrack v7 server to Minecraft 1.4.7 with Forge 6.6.0.515, keeping all of the old mods and configs, and I still get these rendering errors. I also tried upgrading MindCrack v8.0.1 to Forge 6.6.0.518. It seems like the issue is improved, but not to the point of the old server. I haven't been able to do a more quantitative test of the differences between modpack versions and Forge versions yet.
