Why is there delay before plugins take effect?

I’ve noticed that once there is quite a few tracks going on, that plugins, particularly reverbs, take a while to kick in after being applied. Sometimes it feels like a minute or even more.
This seems to be the case with all DAWs on all platforms.
What is the reason for this?

I find some Plugins (u-he Linux native and Fabfilter *used with yabridge) don’t start processing sound when added to a track until they are double-clicked to open their GUI, is this what you mean?

I dont think it’s that Glen. This is something Ive experienced for many
years starting when I used the big name DAWs on Mac.
But now you mention it, I will do a test of what you have described in
Ardour and see if that is in fact what is happening.
Many thanks

I have never experience this myself.
Do you have example? Which plugin(s) exhibit this behavior?

If this only affects commercial/proprietary plugins it could well be that the issue is that the license check happens on the plugin GUI, or perhaps some online service has to be contacted?

I just did a test with a new session with 8 sfizz tracks and 3 dragonfly reverbs and found no delays. the dsp was under 20%.
Next proper big session, I will check again and post the results.
It could just be my perception, but last week I was working with a friend on his macbookpro using logic pro, we seemed to be experiencing this phenomenon and I feel like it was happening to me in an ardour session last night with midi and audio tracks, quite a few (mixed fx) plugins and dsp over 25%.

Does your reverb, by any chance, have a “predelay”? If so, try setting it shorter. “Room size” might habe a similar effect with some plugins. Generally reverb/echo effects need some input and time to build up.