Yes, in those simple terms, it would be 1) simpler to coordinate multiple threads in the same process compared to multiple processes, and 2) more efficient too.
1 is true because any system you might use to communicate between processes would involve an IPC layer and with a single process, you could use an identical system with the IPC removed.
2 is true because multiple threads in the same process can communicate directly without requiring IPC (which requires data serialization, shared memory for performance, and more system calls). i.e, sharing data has much higher cost using IPC. You can skip some serialization if you use shared memory, but you still need to set that up and manage shared buffers. (Different applications might share data in different ways so that the IPC overhead becomes negligible, but non-IPC could always be implemented faster.)
But the benefit gained from using multiple processes for a web browser (that is essentially a VM for running untrusted JS/web code) is security in the face of bugs. It comes from how operating systems provide tried-and-tested process isolation. If sites from separate origins are rendered or parsed in their own process, some types of browser bugs are harder to exploit to gain access to data from other origins. And for bugs that just cause crashes without security implications, you can crash a single tab and let other tabs continue, let the main UI continue.
On its own, that is good, but not great.
The big benefit is that the big 3 operating systems that browser developers are concerned with provide some level of per-process sandboxing. For example, one can launch a process with a limited set of privileges such that the only thing that process is allowed to do is communicate via IPC and compute stuff. The operating system enforces this for you. Provided you have a strong IPC layer, a process with such limited privileges can do very little damage if it is compromised.
Developing a strong IPC layer is not extremely difficult as it essentially boils down to reading a stream of messages from an untrusted source and detecting invalid messages without compromising yourself.