The historical Yahoo servers are now slow, and unfortunately, there is no other option to improve it from our end. Here are some options:
1) Find other solutions than using Yahoo (Historical), like update daily so you don't need to rely on getting historical prices. Yahoo is still very fast, but Yahoo (Historical) is not. You can
automate with a command line the daily price retrieval.
2) Use other historical quote servers. See details/options here:
https://www.fundmanagersoftware.com/intrtv.htmlStooq is very good, but may require you to change some symbols, and doesn't support mutual funds. If you need historical fund pricing, check out Financial Times - Funds (1M Historical).
3) If you need to use Yahoo (Historical), make smaller requests. Right mouse click on a selection and use "Retrieve Selected Historical Prices". Retrieve only a smaller number at a time. Turning off the options to retrieve splits/distributions will also help.