Your comment implies that a network hop between two VPCs is inherently slow. My understanding is the VPCs are akin to a network encryption isolation boundary but should not meaningfully slow down network transfers between each other..

I think you may be conflating the fact that across two VPCs you may be slightly more likely to be doing a cross availability zone or potentially even cross region network hop? I just think it's important to be on the pulse of what's really going on here