Most cryptocurrencies today are mined with specialized hardware targeted at crypto mining, or a Graphics Processing Unit (GPU), which means the CPU based 'Meltdown' vulnerability has no impact on performance. However, that's not the case for all Cryptocurrencies. Monero is in the top 20 currencies, and it supports mining via CPU. The reason that is concerning to Monero miners is because the CPU is affected by the Meltdown vulnerability. Patches to fix 'Meltdown' reportedly can have an impact on the performance of CPU's, unfortunately.
Intel made a statement about the performance impact of Meltdown security patches:
"Any performance impacts are workload-dependent, and, for the average computer user, should not be significant and will be mitigated over time"
That can be a big concern for anybody using their CPU's to mine cryptocurrency, or any other CPU heavy tasks. Even if the performance loss was between 5-30%, it could prove to be a substantial issue to miners who are trying to be as efficient as possible. Leaving Meltdown unpatched isn't an option, however, as it leaves a computer very vulnerable to hackers. (passwords and private information can potentially be extracted) Intel does mention that the performance impacts will be reduced over time, so it may not be a long-term issue.
An author over at Motherboard apparently reached out to Coinhive, a browser-based mining service, and received a statement about the performance impact:
"It is our understanding that the Meltdown patch should not affect mining speeds much or at all," the Coinhive spokesperson wrote. "The Meltdown patch only slows down system calls, but Monero runs entirely in user space. It doesn't need to make any system calls."
Fortunately, it does sound like the issue should have a very low impact (if any) on those mining Monero