Heart rate monitoring always in delay

No no I don't mean Ecg.

I mean as an example: when I'm lifting a weight I notice my heart beat is around 100 bpm but the watch says it's 68.

Than suddendly it bounces (after about 10 seconds) to 120.

So it's not aligned.

Is there something I have to adjust in the settings? 

all garmin watch works this way with the wrist HR sensor. 

the only workaround is to use a chest strap connected to the watch.

A few things I've found over the years...

Optical HR will always have a short delay that you are describing. I find that it's no problem for biking (aside from interval sprints on a trainer). 

For High intensity interval training (plyometrics) where I really need to base my moves on HR numbers, the best thing is to keep a chest strap for those specific workouts and connect it to your watch. 

Wrist based HR can also be a problem with certain types of weight training.  Depending on the moves, you might have the sensor pull away or move on your skin which can cause hiccups or delays.  I have issues mountain biking since the watch can bounce around on rough terrain.

-I use the watch optical sensor for daily HR monitoring, sleep, light workouts.  

-I use an arm-based optical HR sensor for biking and weight training. 

-I do keep a chest (ECG) [not referring to ekg] monitor for really high intensity interval workouts where there is a need for real-time HR with no delay.  I avoid it whenever possible because I just don't like wearing it, and they have their own issues (they can be off in the beginning of a workout if they're dry, etc...)

-To clarify, when I use another sensor, I just pair it with my watch and use it as I normally would.