Honestly, I went through the same dilemma when I first picked up my fp L for landscape work. I ended up sticking with the Atomos Ninja V 5-inch 4K HDMI Recording Monitor and I have been very satisfied with that choice for over a year now. Reliability is basically the most important factor when youre out on a professional shoot and the Ninja V is a tank. While the menu control on the Portkeys sounds convenient on paper, I heard too many reports about firmware glitches and that specific lag issue you mentioned. When youre lining up architectural shots in a city like Chicago, you really dont want to be fighting your gear or wondering if the signal is delayed. The weight of the Ninja V can be a bit of a concern on such a tiny camera body but it works well if you mount it correctly. I use the SmallRig Sigma fp Series Cage 2271 and it distributes the weight perfectly fine for handheld or tripod use. Also, the 1000-nit brightness on the Ninja V is fantastic for outdoor shoots. No complaints about visibility even in direct sun. Plus, you get the benefit of recording to a Samsung 870 EVO 1TB SATA III SSD which is way more reliable than SD cards for long sessions. Staying with the industry standard pays off because it just works every single time. No lag, no drama... just a solid workflow.

Stumbled upon this late but regarding what #2 said about weight balance being everything, thats 100% correct. The fp L is such a unique little camera that putting a heavy recorder on top just feels wrong. Tried a bunch of setups over the years and keeping the footprint small is the only way to go for architecture shoots where youre moving around a lot. Here are some options Ive tested that dont have the Portkeys lag issues:

• Blackmagic Design Video Assist 5 3G
• Build quality is fantastic and the menus are way more intuitive than Atomos.
• SmallHD Focus 5-inch On-Camera Monitor
• Really light and basically the best choice for daylight viewability when youre outside.
• Feelworld F5 Pro V2 5.5 Inch Touchscreen Monitor
• Cheap and cheerful. Its mostly plastic so it wont weigh the rig down. Ngl the fp L micro HDMI port is the weakest link here. Definitely grab a cable clamp or youll regret it halfway through your Chicago shoot...

Like someone mentioned, weight balance is everything. I spent years trying to make those menu-control monitors work, but the lag always felt disconnected. For my city shoots, I switched to a super thin five-inch panel without all the recording guts. It keeps the rig light so the tripod head doesnt creep during long exposures. You really dont want a massive brick on top when you're tilting up for those tall skyscrapers.