The more you compress the longer and more CPU intensive it is to decompress
I believe this is becoming less and less true with modern algorithms. Take for example ZSTD: while the compression speeds differs by several orders of magnitude between the fastest and slowest modes, the decompression difference is only about 20%. The same holds true for flac, where the decompression speed is pretty uniform across all compression levels.
These algorithms probably aren’t used by repacked like fitgirl (so your answer is generally correct in the context of repacks). I do believe it is still interesting to see these new developments in compression techniques.