The LokiCheckout extensions mainly replace the regular Magento 2 checkout with a faster frontend tooling. When it comes to heavy loads, the custom JavaScript architecture, the bundled AJAX updates and the throttling of the AJAX queue, make sure that less calls to the server are needed. Still, if your Magento checkout is under heavy load (like with Black Friday or specific mailings), it makes sense to add additional tuning. This is out-of-the-scope of the LokiCheckout, because tuning is done mostly on the server-level. But this document might serve as a good reference point for what to tune.
Note that the LokiCheckout core creates a separate Loki Component for every single address field. This makes sure that validation and page updates are instant. Yet, it also requires a quote save for every field that is being saved.
Temporarily switch of live API calls for specific shipping methods and/or payment methods. For instance, prefer table rates or carrier quotes. Minimize the number of cart price rules.
Prefer pm=dynamic. Set the max_children properly, depending on the CPU cores available.
Enable the PHP OPcache. Set opcache.memory_consumption to the proper size: For instance, 256Mb for the Magento frontend, higher for the Magento Admin Panel. Set opcache.validate_timestamps=0 and make sure to refresh the OPcache during deployment (for example, using Cachetool).
Set the innodb_buffer_pool_size high enough. Tune other settings over time with a script like mysqltuner.pl.
Use separate Redis databases for the regular cache, the page cache and sessions. Consider setting disable_locking to 1. Consider setting compress_data to 1 only. Consider setting compression_lib to either lzf or zstd if available.
From the perspective of the checkout, adding Varnish does not have real benefits, except for the fact that the Varish Full Page Cache will offload the main PHP server, so that there are more resources for non-cacheable pages like the checkout.