[Bjartr]

Whether or not tabs are in a new process depends on how it was opened. For example, opening a new tab with a control-click or middle-click will result in the new tab share a process with the original, however, right-clicking and selecting "Open in new tab" will result in the new tab having its own process.

[saurik]

I know this, and have even complained about it in the past to Chrome engineers (as it allows an attacker way too much control over getting security sensitive tabs into the same process, further weakening the way overstated security benefits of having separate renderer processes, which is often muddled together with the actual/real benefit of separating processes by capability, as with rendering, networking, windowing, plugins, etc.), but there is also a limited number of processes: as I believe the default is 35, if you have more than 35 tabs open across all windows you are guaranteed to have some tabs sharing a process (by pidgeon hole principle).

Realistically, you just can't use the Chrome task manager to find slow tabs :/. I'd argue that Firefox's recent work in about:performance is actually more useful for this purpose (though isn't very good at dealing with large numbers of tabs that are each using only a small amount of CPU; the real solution to that, though, should just be tab suspension).