I have the new cloud based Lightroom, but I'm noticing it's still taking up a relatively large amount of space on my iMac. I have my original photos backed up on an external hard drive and on Creative Cloud files, but the LR Catalog is still about 50gb. I'm guessing these are mostly smart previews, is there a way to remove them without ruining the catalog or do I just have to live with it?
There are various settings that could be used to help mitigate the impact of Lightroom on your local storage, but some local usage is unavoidable. If you post a screenshot of the Preferences>Local Storage tab we can perhaps mage some suggestions about changes. Use the "Insert Photos" icon to insert your screenshot when replying (so it doesn't have to be downloaded to view it).
One thing I would ask: you said that you have your original photos backed up on Creative Cloud files, is that correct? If so, that would mean that the images are stored twice in the cloud and are thus taking up twice as much space out of your cloud storage limit.
Why not reduce the percentage figure from 25% to something like 10%? That'll still allow LR to use around 40GB of your drive, which should be enough to avoid any slow-downs through continually churning the local caches, but it shouldn't go much above that figure.