There is hardly anything that decodes as fast as FLAC - no lossless audio compressor, at least - and that is part of what FLAC was designed for. Of course, twenty more years of computing power has made both megabytes and CPU cycles much cheaper.
Things to take note of:
* FLAC's "-8" is by no means the slowest setting. It is called "--best" because well, "best among what is useful". Indeed reference FLAC gives you access to "statistical filters" that can improve if you are willing to to pay. Chiptune? Try e.g.
flac --lax -8per15 -A "flatopp;gauss(7e-2);tukey(7e-1);subdivide_tukey(7)" -l 32
and watch the paint dry. It will be slow, and on chiptune a hunch would be that it is the "r" number that improves this much. If you think this is not slow enough, I can offer more "-A" functions to keep your CPU hot.
* The above command will not be FLAC "subset". If you are unlucky, an in-car unit and other hardware players could very well choke on the files you generate. They will be heavier to decode (the "prediction" for each sample will require a history of 32 rather than just 12). Without -l32 it would be much lighter. Without -l32 and with "r8" in place of "r15" it would be within subset. And would still be slow encoding; the "-pe" combines two brute-force elements and is typically not worth it. But with "-r6" and no "-l" switch, decoding will still be about like -7, and that is closer to -0 than to any other lossless codec.
* WavPack's -x settings "do not increase decoding complexity" (well actually, for nitty-gritty reasons they can even improve decoding speeds). They also employ a filterbank for improved compression. -x1 to -x3 are pre-defined filters that are coded in based on knowledge of how music signals typically behave (for CDDA!). -x4 to -x6 will "learn the filtering" from scratch without prior notion of what the music is like, and that is why -x4 can behave so different from -x3, especially on high resolution. Here you got some high resolution numbers: https://hydrogenaud.io/index.php/topic,120454.msg1004848.html#msg1004848 . You see that -x is fairly cheap, and -x4 is much better value for money than -x3; then -x5 and -x6 are expensive.
Both reference encoders can re-encode in-place, so if you first are just ripping CDs you can use a fast setting and then you can run re-encodes overnight / over week-end if that is what you want.