I have no experience with IKEA Tradfri/Home Smart but have you tried to connect one of the speakers by ethernet?

I have this same problem. The remote is utterly useless because there is an initial delay when first turning the knob (after a period of inactivity) of up to 15 seconds before the remote responds to input and the sound level changes. Then, it will often do a runaway volume increase or decrease as though it’s finally implementing, all at once, any input that was delayed. It seems like the remote goes to sleep and then takes way too long to wake up and get the command to the speakers. Is this a problem on IKEAs end or SONOS?

Honestly, it might be neither. It’s possible that it’s a network issue, with the remote taking time to get a connection to your router for any of a various number of reasons. Unfortunately, I’m not overly familiar with these remotes, but will assume it has a battery, and make a suggestion based on that assumption. This procedure is only taking time, and there’s no extra purchases involved, so you can try it when you want, and see if it makes a difference. 

Unplug all Sonos and Sonos/IKEA devices from power, including removing the battery from the remote. While they’re in that state, reboot your router. When the router comes back up, go ahead and plug back in the Sonos and Sonos/IKEA devices in to power, and then place the battery back into the remote. There should be no reason that you need to add the devices back to the system, they should just connect. Give them a couple of minutes to connect and settle down, then test the remote. 

Honestly, I’d be extremely tempted to try a replacement battery, too, on the offhand chance the signal from the remote is ‘weak’ and needs more power to connect properly, but that’s an extra cost step. I’m never confident when I get batteries with a electronics purchase on how long they’ve been sitting in the store before I purchased them. In the grand scheme of things, it shouldn’t be an issue, but I’m paranoid ;)