C6v2 uses a single core QCA9563 mips CPU. Using it as a router, LAN to LAN I managed about 500Mbps. Using it just as a wireless AP with VLANs it manages the same performance as stock, and with ath10k-ct-smallbuffers and with -ct firmware I haven't seen any issues (current uptime 48 days). If you need a gigabit router with OpenWRT, use at least a MT7621 based device with MT7615D wifi and enable hardware flow offloading.
For reverting to stock, can you try the guide?