This was driving me insane for the better part of a day.

Fun fact, Logitech Harmony Hub defaults to talking to Roku devices via network-based remote commands. In theory, it's a really cool idea... until you realize the latency penalties of multiple hops:

Harmony Remote --> Harmony Hub --> Router --> Roku

Depending on a number of factors (not the least of which being WIFI congestion), I was noticing up to a 2-3 second delay between pressing a button on the Harmony remote and having the Roku UI register it. I know, it sounds like nothing but I assure you it can be an eternity. :-)

The solution: Force the Harmony Hub to revert back to using IR blast instead of network commands. Ironically, it's not a configurable option... but there is a way to do it.

Hat tip to this reddit post for describing the process. I'll post it below for internet archiving but again, all credit goes to the original author...

SOLVED:

I ended up fixing it. Apparently once you setup IP capabilities with Roku devices, you cannot override them even if you assign them to the remote or IR via the Harmony app. I tested with an older Roku 2 and got the same result.

Here is what I did.

I removed the Roku from my list of devices.

Then I disconnected the Roku completely from Wi-Fi. I unplugged the Roku as well.

Then from the Harmony app, I added back the Roku as a device. It will try to find the Roku on your Wi-Fi to enable IP capabilities. It will fail obviously since it's disconnected.

When prompted select, "Skip IP Scan" and that will just add the Roku as a regular device w/o IP capabilities.

Once it is added, go into the Hub Assignment settings and enable the Roku for either Mini blaster port you have.

Setup/Re-add your activities involving the Roku before connecting the Roku back to Wi-Fi, or else it will try to automatically enable IP capabilities.

Reconnect Wi-Fi

I noticed for me that it's a lot smoother and more responsive than Wi-Fi. One big thing I noticed that drove me crazy before was any video I would watch, trying to fast forward you could only go through 10 sec increments when holding the button. Now on IR it works just like the OG remote and lets you scrub through videos quickly.

Only downside is you lose access to Roku favorite channels on the remote, but I'd rather have a consistently working and smoother device than convenient channel buttons.