Alibaba recently said their XT910 was slightly faster than the A73. Since the first actual A73 launched in Q4 2016, that would imply they are at most 4 years behind.

SiFive's U8 design from last year claimed to have the same performance as A72 with 50% greater performance per watt and using half the die area. Consider how popular the Raspberry PI is with it's A72 cores. With those RISCV cores, they could drastically increase the cache size and even potentially add more PCIe lanes within the same die size and power limits.

Finding out new things takes much more time than re-implementing what is known to already work. As with other things, the 80/20 rule applies. ARM has caught up a few orders of magnitude in the past decade. RISCV can easily do the same and give the lack of royalties. Meanwhile, the collaboration helps to share costs and discoveries which might mean progress will be even faster.

I'm not saying Apple can drop-in RISC-V front-end to their silicon and call it a day, but you get the idea.

Sifive has a pretty decent chance at making performant chips within next few years.