Probably depends on how many mods you have installed and whether you are using ini tweaks to enhance visual quality (e.g. a higher uGridsToLoad value). A vanilla Oblivion pretty much should run at 90fps at all times, but it’s not overly difficult to make the game CPU bound with mods and ini tweaks, in which case even the fastest graphics card doesn’t change anything.
Not sure about the blocking issue. Oblivion is one of the games I played myself a fair bit. Don’t recall anything like that, but I’ll check it.
BTW: You can make it look fantastic (well, as far as a game that old can look fantastic) by forcing antialiasing in your graphics driver control panel. That way you can use AA and HDR rendering together, which the game normally does not allow.