REX contains the option to select 512X512 or 1024X1024 resolution Clouds.
If the "Cloud" load is too heavy, start the Sim and choose Clear Weather, or Custom Weather, and save the flight, and make it the Default Flight.
This will retain all the other installed REX effects, with your choice of clouds...!
(Re-load REX after selecting a Default Clear Sky, and and un-select Clouds?)...

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Paul...G-BPLF...FS 2004...FS Nav...REX for FS2004... 8-)...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.