If you render in a room then the HDRI light is going to enter only through any openings, meaning that less light will get in (making the result darker) and that areas not directly in line with the openings are relying on light paths bouncing one or more times to reach them slowing the process down. Adding lights (actual, local lights or ghost lights) inside the enclosed space can help - note that disant lights are not local, the avatar is just a guide to orientation but the light iss till coming from without and so will be blocked by the interior model.