TL;DW version: sea of nodes requires a scheduling pass, which was taking 20% of their compilation time. But it sounds like there's a lot of legacy baggage, so ...
My GCM doesn't take 20% of my compile times last I checked but even so, V8 is gonna be in a special camp because they're comparing against a compiler that doesn't do much code motion. LLVM does a lot of code motion so that "20%" is still being paid by LLVM during their hoisting and local scheduling passes.
V8 has been moving away from sea-of-nodes. Here's a video where Ben Titzer is talking about V8's reasons for moving away from sea-of-nodes: https://www.youtube.com/watch?v=Vu372dnk2Ak&t=184s. Yasser, the author of Tilde, is is also in the video.