The most common mistake people make leading to the lowered performance is differences in resolution. SteamVR lets you set the global resolution and also per-app resolution. But these settings are 100% ignored when bypassing SteamVR via OpenComposite and PimaxXR.
You can still use the rendering quality slider in Pitool/Pimax Client to alter the resolution (PimaxXR will read that), or perhaps the Override resolution in OpenXR Toolkit.
Btw here is my write-up instructions for OpenComposite (not specific to Pimax though):
I will add a gotcha for the resolution thing, since it’s not obvious. Hopefully this is your issue.