This slow update issue happened since RVT was introduced in early 2021, it’s not related to textures (except the RVT itself of course), landscape size, etc. but to how the engine is streaming this virtual texture, even more when the virtual texture resolution is high (like 4MiTexels we both use).
Is your project stored on a HDD, a SSD, a M.2 drive? Because as it’s a streaming feature, the faster your drive is, the faster the virtual texture will be updated (same for RAM/VRAM bandwith I guess), even it shouldn’t be as slow.
This issue doesn’t happen on games (other engines than UE), using adaptive virtual textures, installed on HDDs.