From personal experience using occluders to optimize props separately from visgroups (each being divided by hint/skip brushes) demands computation. Too many occluders causes the game to perform an overflow of intricate calculations all trying to hide hundereds of props within hidden areas. I had to fix this in my snow version of ravenholm, where my attempt to mitigate low fps using occluders made framerate worse. I solved it by sectioning the map using skybox brushes to hide both props and brushes simultaneously, using fewer hint/skip brushes, and replacing occluders with area portals for large interior spaces. Larger chunks of the map can neatly vanish, smoothing the fps as the player transitions among viisgroups. Hopefully I can help!

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