No they can't. That doesn't even make sense as a claim regarding bandwidth since SQLite doesn't use any, but please re-read what I said about being a 1% or 5% difference in speed. Not 10x.
Same-core context switching costs a few microseconds.
Going across core complexes can cost tens to hundreds of microseconds.
These figures are several orders of magnitude (5-6) slower than L1 access on the same thread.
Communication between processes is negligible compared to all of the sequential disk/SSD accesses and processing required for executing queries.
The database isn't stored in L1 and communication isn't taking hundreds of microseconds. I don't know where you're getting your information.
The fact that SQLite is in-process is primarily about simplicity and convenience, not performance. Performance can even be worse, e.g. due to the lack of a query cache.