Approximately since I started TR workouts on zwift (3 weeks ago) I’ve noticed it takes a couple of mins for ERG to kick in. It shows as turned on, but doesn’t restrict power output. Is this a Zwift issue? Wahoo (using a kickr core)? I’m sure it’s not a TR issue! E.g shown of 163w in 120w warmup.
Probably a trainer issue. Check that the wahoo app isnt running and also recalibrate your trainer. I’ve found TR workouts in zwift that you can produce higher power in erg mode than the target power, in TR it seems the trainer adjusts more and doesnt let you overshoot.
Had same thing happen to me with Saris H3 when I stopped and restarted pedaling during warm up to adjust a fan. Tried adjusting intensity a few times before giving up and restarting workout. Thankfully this was at the start. Now I don’t dare stop pedaling for any reason! 
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I’ve been doing TR workouts in Zwift since the day after the integration was announced (Tacx Neo 2 and iPad) and have never had an issue. Erg starts immediately, and changes promptly at each arch. You might try rebooting your router and ensuring all of your equipment is paired correctly and calibrated.
I believe this relatively common in Zwift. Try changing the bias a couple times (i.e. 101-102% to change the power target and then back to 100%) that might trigger the ERG again.
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Probably not an issue in your case, but some have had issues when multiple apps/devices were trying to control the trainer, e.g. Zwift and a Garmin Edge. If using a Garmin Edge to record a ZWIFT or TR workout, pair the trainer as a PM (and speed and cadence if using those) rather than as a trainer.
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ERG mode performance in Zwift is a known issue.
I’d still make sure that you’re not getting signal interference from other devices and that your trainer is only connected to Zwift.
Let me know if you have issues that block you from getting your workouts in!
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If you pause your workout or disable ERG mode that can cause some major wrenches in Zwift. You’ll have to ride around the power to get it to reengage which can be difficult.
As others have said there can be issues where Zwift and some other app/computer are both trying to control the power output.
I use Zwift with a Zwift Hub and aside from it taking about 5 seconds to get to the appropriate power level when an interval changes it works incredibly well.
It usually happens when ERG mode takes control too late after a Bluetooth reconnect. Try switching to ANT+ or disable PowerMatch for a test. It fixed the same lag on my Kickr Core.