Increasing length of session makes Pro Tools lag?

[ArkMusic]

Hi! Been an avid lurker on these forums for some time, and this is my first post and question.

I am working on a track that has a lot of MIDI/VI's, over 25 or more including drums. Also I have around 5 tracks that are Mono Audio tracks.
I started out building up each section of the song, about 4 in total. Yesterday I went to arrange it properly and structure the song, so it starts with an intro, then verse parts and chorus parts etc..

The thing is: I had no lag issues before I started copying sections and making the session longer. When each section had just one instance (one verse part, one chorus part), everything was playing smoothly, but as soon as I duplicated them (two verse parts and three chorus parts), my session became laggy, audio crackles constantly, and theres a delay when pressing play.

Does anyone here have any experience with this problem? Why is session length more taxing on my computer than number of VI's? Because I didn't add any new instruments when I started structuring the song, I just duplicated midi data. And is there any way to mitigate these performance issues?

I have 16 Gbs of RAM, an intel i7 5820 @4.2 Ghz, latest Windows 10 update, running latest Pro Tools I think, 2021.6.