No. In fact, it may improve performance, by virtue of transferring less data from slower I/O devices. Yes, slower, even if it's a SSD.
I'm skeptical of this actually being the case in any real-world scenarios. There have been a number of tests of running games from a RAM disk vs an SSD, with precious little difference in load times.
The key is the data sent to the cpu and decompressed, makes up for the stall from hitting memory or i/o. Comparing ram vs ssd is the wrong comparison to make, with both you're hitting stalls due to memory. You want to compare reads of uncompressed versus compressed with the note that (and i'm just making numbers up with this analogy as i'm about to sleep), 900KiB of compressed data in, 2MiB of data out. 1.1MiB bonus and yes I'm assuming huge compression but for times your cpu is idle it makes perfect sense.
And yes, lz4 compression on things like movies still helps. I shaved off over 200GiB on my home nas with zfs.
Also useful if you don't have fast internet for your gaming PC, but access to it somewhere else.
Wait, you run the Steam apps from a compressed folder? Doesn't that kill performance?
Regardless, I have long felt that the right way for Steam to make this work would be to have, in addition to the "Download" and "Delete" tools, an "Archive" button that moves it to a specified path (by default, on the install drive, but configurable to another drive or a NAS) and compresses it - with whatever compression they want.
I want my Steam apps uncompressed on my SSD. I don't have room on my SSD for hundreds of gigabytes of Skyrim textures that I haven't played in 6 months. But if I delete the app, then I have to wait hours (and cause Steam some expense) to download it again.
Not everyone has multiple drives or a NAS, but a local archive would definitely be useful.