Wifi slow and delays with wifi power save on
It looks like you've already come across one solution: turning off Wi-Fi power saving. That option puts the Wi-Fi chip into a low-power state sometimes. This reduces power use, but the trade-off is that next time you want to use Wi-Fi, you have to wait a fraction of a second for the chip to power on again.

This page from the official Android documentation explains about Wi-Fi power states. It's targeted at app developers, so they can optimize the usage pattern of their apps to save power. Even so, it's easy to follow, and might be interesting for other users to understand how Android tries to save power.