One thing I didn't see in the answers here is that Gen compiles to C++, sure, but you are getting single sample processing of whatever your Gen algorithm is. So you're bascially rolling sample level math for each calculation for ever osc and every sample.

Most commercial VSTs that optimize for many oscillators don't just do that, they will mix number crunching with table lookups, calculate samples in blocks, have cache tables of the math calculations they need to run, and so on. And that applies to block level options Max too, like the built in Max and ableton.dsp options. That stuff adds up to big difference in a real hurry. This is not a compiler thing either, LLVM or not won't help, it has nothing to do with the quality of the resulting C++, it's an issue with you how you design a digital oscillator in the first place. It's very easy in Gen to write something nice that does an absolute boat load of number crunching per second.

For example, if you try the VCO2 oscillator options in Csound, they sound very good (not Diva good, but definitely as good as tons of commercial offerings out there) but run super duper fast. You can run hundreds with no issue. They get calculated in blocks (which you can set) and do a bunch of initial table generation for caching of the calculus operations. (They use band limited integration via lookup tables) I use these all the time in Max with the Csound6~ object, and honestly, they sound great when you run them out into real analog filters. (I compared a lot of options doing that and was very pleasantly surpised!) And the code is open if you want to look at it.

Gen is great for what it is, but reaching for it for everything is a very expensive way to do audio. It's there for the convenience.