I have a large catalog and every time I open LR my storage gets ~20GB added to the used space because of LR.
By @AndreiFurnea
That might be because if you browse different photos than you did last time, new previews are generated, and cached in the …Previews.lrdata file next to the catalog. Or, if you edit more photos, the edit data may be added to the Camera Raw cache which is always on the system volume by default. For these reasons I always keep at least 100GB free on my system volume.
The …Previews.lrdata file can grow over time, for example to 20 or 50GB. When it gets really large I throw it out; because it’s just a preview cache Lightroom Classic will rebuild it automatically. If you move your catalog folder to another volume, the growth in the preview cache will no longer affect your system volume.
The Camera Raw cache size can be limited in Lightroom Classic preferences, and if it is, it won’t keep growing past the limit you set in the preferences. There is also a video cache in preferences that you can limit.
In order to avoid that I want to create a new catalog on an external storage and add all my external hdd's photos to it, but I'm not sure if I'll see a big drop in the speed of how LR will work.
By @AndreiFurnea
If the catalog is on external storage, then it and the previews are both there, so the access time will depend on whether you are using a hard disk drive or a solid state drive. If you use a HDD, there might not be a noticeable drop in loading originals (because they are not read very often), but there may be a noticeable slowdown when reading/writing to the catalog and preview file (because those are constantly read and written to). So originals are OK on an external HDD, but if you are also going to store the catalog folder on the external volume then it would be better for it to be an SSD.
There are different speed levels of SSDs. For photo editing, the cheapest SSD option (SATA interface connected with USB 3 or later) is fine, and 5x faster than a hard drive. You can spend more money on NVMe SSDs; if you do, a 1000 megabytes per second (MB/sec) SSD connected by 10Gbps USB 3 is also fine, and may be useful if you also edit video sometimes. For photo editing, it’s a waste of money to get the highest level (NVMe SSD at 3000+MB/sec connected by 40Gbps USB 4 or Thunderbolt), but if you do pro level video editing that might be a good idea. (For comparison, internal Mac SSD speed can be 1300–7400MB/sec depending on the model.)