One possibility is that the timing is usefully different with Electron - the products were out and getting traction (and presumably in Slack's case revenue) long before the performance hacks were conjured.

I don't know if the total amount of work over time sums to approach that of a portfolio of native apps, but even if that's the case, there's a world of difference between spending extra time improving a product that's already out in the world and developing the first cut.